Previously we were using the cull rect of the SkPicture to size the GPU buffer,
but Skia is too smart and shrinkwraps the cull rect, which meant we weren't
sizing the GPU buffer to the right size.
Fixes#814
Previously there was a race condition when updating the size of the GLSurface.
Now we explicitly update the size of the surface to match what we expect.
Fixes#610