gl_engine: support cross compile GL backend into WASM

Support cross compile the GL backend code into WASM.
The code needs WebGL 2.0 API so, the compile flags contains `MAX_WEBGL_VERSION` and `FULL_ES3`
Also add binding code to initialize the WebGL context and GLCanvas.

cross/wasm32_gl.txt

@@ -0,0 +1,21 @@
+[binaries]
+cpp = 'EMSDK:upstream/emscripten/em++.py'
+ar = 'EMSDK:upstream/emscripten/emar.py'
+strip = '-strip'
+
+[properties]
+root = 'EMSDK:upstream/emscripten/system'
+shared_lib_suffix = 'js'
+static_lib_suffix = 'js'
+shared_module_suffix = 'js'
+exe_suffix = 'js'
+
+[built-in options]
+cpp_args = ['-Wshift-negative-value', '-flto', '-Os', '-fno-exceptions']
+cpp_link_args = ['-Wshift-negative-value', '-flto', '-Os', '-fno-exceptions', '--bind', '-sWASM=1', '-sALLOW_MEMORY_GROWTH=1', '-sEXPORT_ES6=1', '-sFORCE_FILESYSTEM=1', '-sMODULARIZE=1', '-sEXPORTED_RUNTIME_METHODS=FS', '-sMAX_WEBGL_VERSION=2', '-sFULL_ES3']
+
+[host_machine]
+system = 'emscripten'
+cpu_family = 'wasm32'
+cpu = 'wasm32'
+endian = 'little'

src/bindings/wasm/tvgWasmLottieAnimation.cpp

@@ -27,6 +27,9 @@
 #ifdef THORVG_WG_RASTER_SUPPORT
     #include <webgpu/webgpu.h>
 #endif
+#ifdef THORVG_GL_RASTER_SUPPORT
+    #include <emscripten/html5_webgl.h>
+#endif
 
 using namespace emscripten;
 using namespace std;
@@ -155,6 +158,54 @@ struct TvgWgEngine : TvgEngineMethod
     }
 };
 
+struct TvgGLEngine : TvgEngineMethod
+{
+    uintptr_t context = 0;
+    ~TvgGLEngine() override
+    {
+    #ifdef THORVG_GL_RASTER_SUPPORT
+        if (context) {
+            Initializer::term(tvg::CanvasEngine::Gl);
+            emscripten_webgl_destroy_context(context);
+            context = 0;
+        }
+    #endif
+    }
+
+    Canvas* init(string& selector) override
+    {
+    #ifdef THORVG_GL_RASTER_SUPPORT
+        EmscriptenWebGLContextAttributes attrs{};
+        attrs.alpha = true;
+        attrs.depth = false;
+        attrs.stencil = false;
+        attrs.premultipliedAlpha = true;
+        attrs.failIfMajorPerformanceCaveat = false;
+        attrs.majorVersion = 2;
+        attrs.minorVersion = 0;
+        attrs.enableExtensionsByDefault = true;
+
+        context = emscripten_webgl_create_context(selector.c_str(), &attrs);
+        if (context == 0) return nullptr;
+
+        emscripten_webgl_make_context_current(context);
+    #endif
+
+        if (Initializer::init(0, tvg::CanvasEngine::Gl) != Result::Success) return nullptr;
+
+        return GlCanvas::gen();
+    }
+
+    void resize(Canvas* canvas, int w, int h) override
+    {
+    #ifdef THORVG_GL_RASTER_SUPPORT
+        emscripten_webgl_make_context_current(context);
+
+        static_cast<GlCanvas*>(canvas)->target(0, w, h);
+    #endif
+    }
+};
+
 
 class __attribute__((visibility("default"))) TvgLottieAnimation
 {
@@ -171,6 +222,7 @@ public:
         errorMsg = NoError;
 
         if (engine == "sw") this->engine = new TvgSwEngine;
+        else if (engine == "gl") this->engine = new TvgGLEngine;
         else if (engine == "wg") this->engine = new TvgWgEngine;
 
         if (!this->engine) {

src/renderer/gl_engine/meson.build

@@ -26,6 +26,9 @@ source_file = [
 if host_machine.system() == 'darwin'
     gl_dep = declare_dependency(link_args: ['-framework', 'OpenGL'])
     compiler_flags += ['-Wno-deprecated']     #for deprecated opengl
+elif cc.get_id() == 'emscripten'
+    target_opengles = true
+    gl_dep = dependency('GLESv3', required: false)
 else
     #find a opengl library with fallbacks
     gl_dep = dependency('GL', required: false)

src/renderer/gl_engine/tvgGlCommon.h

@@ -42,8 +42,11 @@
 #include "tvgCommon.h"
 #include "tvgRender.h"
 
-
-#define GL_CHECK(x) \
+#ifdef __EMSCRIPTEN__
+    // query GL Error on WebGL is very slow, so disable it on WebGL
+    #define GL_CHECK(x) x
+#else
+    #define GL_CHECK(x) \
         x; \
         do { \
           GLenum glError = glGetError(); \
@@ -52,6 +55,7 @@
             assert(0); \
           } \
         } while(0)
+#endif
 
 enum class GlStencilMode {
     None,

wasm_build.sh

@@ -17,6 +17,9 @@ if [ ! -d "./build_wasm" ]; then
     elif [[ "$BACKEND" == "sw" ]]; then
       sed "s|EMSDK:|$EMSDK|g" ./cross/wasm32_sw.txt > /tmp/.wasm_cross.txt
       meson -Db_lto=true -Ddefault_library=static -Dstatic=true -Dloaders="all" -Dsavers="all" -Dthreads=false -Dbindings="wasm_beta" --cross-file /tmp/.wasm_cross.txt build_wasm
+    elif [[ "$BACKEND" == "gl" ]]; then
+      sed "s|EMSDK:|$EMSDK|g" ./cross/wasm32_gl.txt > /tmp/.wasm_cross.txt
+      meson -Db_lto=true -Ddefault_library=static -Dstatic=true -Dloaders="all" -Dsavers="all" -Dthreads=false -Dbindings="wasm_beta" -Dengines="gl" --cross-file /tmp/.wasm_cross.txt build_wasm
     else
       sed "s|EMSDK:|$EMSDK|g" ./cross/wasm32_wg.txt > /tmp/.wasm_cross.txt
       meson -Db_lto=true -Ddefault_library=static -Dstatic=true -Dloaders="all" -Dsavers="all" -Dthreads=false -Dbindings="wasm_beta" -Dengines="wg, sw" --cross-file /tmp/.wasm_cross.txt build_wasm