WebGL 2: add readPixels API to read pixels into pixel pack buffer

Source/modules/webgl/WebGLRenderingContextBase.h

Index: Source/modules/webgl/WebGLRenderingContextBase.h
diff --git a/Source/modules/webgl/WebGLRenderingContextBase.h b/Source/modules/webgl/WebGLRenderingContextBase.h
index c6cb76eacb089a3c6ce423667f5ee65a41410ce6..c83e9797b5d5001382374c7a60f70452a5c51122 100644
--- a/Source/modules/webgl/WebGLRenderingContextBase.h
+++ b/Source/modules/webgl/WebGLRenderingContextBase.h
@@ -122,6 +122,33 @@ struct FormatTypeCompare {
     }
 };
 
+// ScopedDrawingBufferBinder is used for ReadPixels/CopyTexImage2D/CopySubImage2D to read from
+// a multisampled DrawingBuffer. In this situation, we need to blit to a single sampled buffer
+// for reading, during which the bindings could be changed and need to be recovered.
+class ScopedDrawingBufferBinder {
+    STACK_ALLOCATED();
+public:
+    ScopedDrawingBufferBinder(DrawingBuffer* drawingBuffer, WebGLFramebuffer* framebufferBinding)
+        : m_drawingBuffer(drawingBuffer)
+        , m_readFramebufferBinding(framebufferBinding)
+    {
+        // Commit DrawingBuffer if needed (e.g., for multisampling)
+        if (!m_readFramebufferBinding && m_drawingBuffer)
+            m_drawingBuffer->commit();
+    }
+
+    ~ScopedDrawingBufferBinder()
+    {
+        // Restore DrawingBuffer if needed
+        if (!m_readFramebufferBinding && m_drawingBuffer)
+            m_drawingBuffer->restoreFramebufferBindings();
+    }
+
+private:
+    DrawingBuffer* m_drawingBuffer;
+    Member<WebGLFramebuffer> m_readFramebufferBinding;
+};
+
 class MODULES_EXPORT WebGLRenderingContextBase : public CanvasRenderingContext, public Page::MultisamplingChangedObserver {
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(WebGLRenderingContextBase);
 public:
@@ -250,7 +277,7 @@ public:
     void linkProgram(WebGLProgram*);
     void pixelStorei(GLenum pname, GLint param);
     void polygonOffset(GLfloat factor, GLfloat units);
-    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels);
+    virtual void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels);
     void renderbufferStorage(GLenum target, GLenum internalformat, GLsizei width, GLsizei height);
     void sampleCoverage(GLfloat value, GLboolean invert);
     void scissor(GLint x, GLint y, GLsizei width, GLsizei height);
@@ -850,6 +877,10 @@ protected:
     // Generates INVALID_OPERATION and returns false if the combination is unsupported.
     bool validateReadPixelsFormatTypeCombination(GLenum format, GLenum type, GLenum readBufferInternalFormat, GLenum readBufferType);
 
+    // Helper function to check parameters of readPixels. Returns true if all parameters
+    // are valid. Otherwise, generates appropriate error and returns false.
+    bool validateReadPixelsFuncParameters(GLsizei width, GLsizei height, GLenum format, GLenum type, long long bufferSize);
+
     virtual GLint getMaxTextureLevelForTarget(GLenum target);
 
     // Helper function to check input level for functions {copy}Tex{Sub}Image.