gdkglcontext: Track whether to use GL_TEXTURE_2D or GL_TEXTURE_RECTANGL_ARB
opengl: Use proper blend function for premultiplied alpha
opengl: Use GL_TEXTURE_2D rather than GL_TEXTURE_RECTANGLE_ARB when possible

This is more standard, and most driver support non-power-of-2 TEXTURE_2D
these days. We fall back for ancient drivers.
gtk3-demo: Add a frame to the sidebar

With overlay scrollbars, there's no visible separation between
the sidebar and the content otherwise.