This change is needed to allow qtdeclarative to build with change
https://codereview.qt-project.org/#change,28334

 as OpenGL ES 2 and
the EXT_sRGB do not define the symbol GL_FRAMEBUFFER_SRGB.

Also use symbols defined by OES extensions where needed.

The #defines will be removed in a follow-up commit once 28334 has
been merged.

Change-Id: I1c4e5297c29ecf723463da7fbfe353628c4c35ef
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>