gl: avoid using an extra FBO pass for formats that requires a shader at upload time

Submitted by Julien Isorce @cap

Link to original bug (#730927)

Description

Currently "videotestsrc ! video/x-raw, format=I420 ! glimagesink" uses a
shader to do the color convertion, it's fine but we use a FBO for that which
can be avoided. Indeed we could draw directly to the window surface.