gl_engine: fix always-true clear flag

The clear flag specified in Canvas::draw is ignored when set to false,
since GlRenderer::mClearBuffer is never explicitly reset to false.

This commit ensures that mClearBuffer is reset to its default (false)
after being used, so that the clear operation behaves correctly per frame.

- Issue: #1779

src/renderer/gl_engine/tvgGlRenderTask.cpp

@@ -195,8 +195,6 @@ void GlComposeTask::run()
 {
     GL_CHECK(glBindFramebuffer(GL_FRAMEBUFFER, getSelfFbo()));
 
-    // clear this fbo
-    if (mClearBuffer) {
     // we must clear all area of fbo
     GL_CHECK(glViewport(0, 0, mFbo->getWidth(), mFbo->getHeight()));
     GL_CHECK(glScissor(0, 0, mFbo->getWidth(), mFbo->getHeight()));
@@ -211,7 +209,6 @@ void GlComposeTask::run()
 
     GL_CHECK(glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT));
     GL_CHECK(glDepthMask(0));
-    }
 
     GL_CHECK(glViewport(0, 0, mRenderWidth, mRenderHeight));
     GL_CHECK(glScissor(0, 0, mRenderWidth, mRenderHeight));

src/renderer/gl_engine/tvgGlRenderer.cpp

@@ -853,6 +853,9 @@ bool GlRenderer::sync()
 
     clearDisposes();
 
+    // Reset clear buffer flag to default (false) after use.    
+    mClearBuffer = false; 
+
     delete task;
 
     return true;

src/renderer/gl_engine/tvgGlRenderer.h

@@ -153,7 +153,7 @@ private:
     } mDisposed;
 
     BlendMethod mBlendMethod = BlendMethod::Normal;
-    bool mClearBuffer = true;  //FIXME: clear buffer should be optional (default is false)
+    bool mClearBuffer = false;
 };
 
 #endif /* _TVG_GL_RENDERER_H_ */