gl_engine: fix FBO validation

At the moment, we use the value GL_INVALID_VALUE
to mark an invalid handle for FBO, which is not correct.
It is more correct to use the value 0,
which signals that the buffer was not created.
Canvas initialization processes have also been adjusted.

src/renderer/gl_engine/tvgGlRenderTarget.cpp

@@ -31,7 +31,8 @@ GlRenderTarget::~GlRenderTarget()
 
 void GlRenderTarget::init(uint32_t width, uint32_t height, GLint resolveId)
 {
-    if (mFbo != GL_INVALID_VALUE || width == 0 || height == 0) return;
+    if (width == 0 || height == 0) return;
+
     mWidth = width;
     mHeight = height;
 
@@ -77,14 +78,16 @@ void GlRenderTarget::init(uint32_t width, uint32_t height, GLint resolveId)
 
 void GlRenderTarget::reset()
 {
-    if (mFbo == 0 || mFbo == GL_INVALID_VALUE) return;
+    if (mFbo == 0) return;
+
     GL_CHECK(glBindFramebuffer(GL_FRAMEBUFFER, 0));
     GL_CHECK(glDeleteFramebuffers(1, &mFbo));
     GL_CHECK(glDeleteRenderbuffers(1, &mColorBuffer));
     GL_CHECK(glDeleteRenderbuffers(1, &mDepthStencilBuffer));
     GL_CHECK(glDeleteFramebuffers(1, &mResolveFbo));
     GL_CHECK(glDeleteTextures(1, &mColorTex));
-    mFbo = GL_INVALID_VALUE;
+
+    mFbo = mColorBuffer = mDepthStencilBuffer = mResolveFbo = mColorTex = 0;
 }
 
 GlRenderTargetPool::GlRenderTargetPool(uint32_t maxWidth, uint32_t maxHeight): mMaxWidth(maxWidth), mMaxHeight(maxHeight), mPool() {}

src/renderer/gl_engine/tvgGlRenderTarget.h

@@ -44,13 +44,13 @@ public:
     void setViewport(const RenderRegion& vp) { mViewport = vp; }
     const RenderRegion& getViewport() const { return mViewport; }
 
-    bool invalid() const { return mFbo == GL_INVALID_VALUE; }
+    bool invalid() const { return mFbo == 0; }
 
 private:
     uint32_t mWidth = 0;
     uint32_t mHeight = 0;
     RenderRegion mViewport{};
-    GLuint mFbo = GL_INVALID_VALUE;
+    GLuint mFbo = 0;
     GLuint mColorBuffer = 0;
     GLuint mDepthStencilBuffer = 0;
     GLuint mResolveFbo = 0;

src/renderer/gl_engine/tvgGlRenderer.cpp

@@ -790,7 +790,7 @@ bool GlRenderer::clear()
 bool GlRenderer::target(void* context, int32_t id, uint32_t w, uint32_t h)
 {
     //assume the context zero is invalid
-    if (!context || id == GL_INVALID_VALUE || w == 0 || h == 0) return false;
+    if (!context || w == 0 || h == 0) return false;
 
     if (mContext) currentContext();