vcl: Add initial support for linear gradient with OpenGL

Change-Id: Iccc12c94bfd68387dfc0161a5fde4f595edda0e1
vcl: Read back OpenGL FBO to create offscreen X11 pixmap

Change-Id: I330e7d62bf31b4a90b5866d9531f073f7c69c92a
vcl: Add support for radial gradients in OpenGL backend

Change-Id: Ie47fb18ae7d5286fe7559c7dffbc54b0856d4d8e
vcl: Use offscreen rendering for native GTK widgets with OpenGL backend

Change-Id: I142e1b34e54a3cbe149e5af9adeba1250a4ca9a9