vcl: Always use the same OpenGL context when drawing in a window

Change-Id: Ief0e947149c133aaa8e81973e088c4df6432bfdc
vcl: Move the painting fence to the OpenGL context

Change-Id: I462e68a1ad7a56fafe57504959cf169a70665f81
vcl: Track the GL context's clip region and update before drawing when needed

Change-Id: Ibec92851dc87f6696ee55a8db10fe160cd97d09c
vcl: Improve precision and performance of clipping when region is a RegionBand

Change-Id: I7a481ba86d03b0eb8f4b456e38cfa89b6cbc209d
vcl: Re-flip native X11 widgets upside down in OpenGL backend

Change-Id: I1819f8e3357dc2f805b0ecebca0659fe073ec611
vcl: Make sure the active framebuffer is the right one before drawing

Change-Id: Icc30bee1d58dbf8f5e7b65ba90cfdf0c9135b464