It is implemented in the way identical to how it works in CPU painter:
1. SampleUnderCorners command saves pixels within corners into a
texture.
2. BlitCornerClipping command uses the texture prepared earlier to
restore pixels within corners.
For each stacking context with an opacity less than 1, we create a
separate framebuffer. We then blit the texture attached to this
framebuffer with the specified opacity.
To avoid the performance overhead of reading pixels from the texture
into Gfx::Bitmap, a new method that allows for direct blitting from
the texture is introduced, named blit_scaled_texture().
With these changes it is now possible to create OpenGL context on macOS
and run GPU-painter. For now only QT client has a CLI param that turns
it on though.
Framebuffer object is allocated using OpenGL's API and is not platform
specific which means it could be used on both macOS and Linux unlike
EGL specific PBuffer.
Before, we were using a feature of OpenGL that allows specifying a
pointer to allocated vertex data without creating VBO and VAO, but as
I found out, it does not work on macOS.
This change introduces GL.h with error check wrappers for all the
OpenGL functions we used so far.
For now, the error check is simply:
`VERIFY(glGetError() == GL_NO_ERROR);`
but that is better than continuing execution after encounting an error.
