Fix documentation of the QOpenGLFunctions::OpenGLFeature enum

It's not really about features already present in OpenGL ES 2 that
might not be present in other OpenGL implementations; for instance,
NPOT textures are not in ES 2, but only available through extensions.

Therefore, change the wording to include optional OpenGL and OpenGL ES
features.

Change-Id: I3822e50789ded10a937e3c5b7b4be9e267265276
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Add a way to test if an OpenGL context has the fixed function pipeline available

Due to the mess done in 3.0/3.1 this is trickier than it should be,
so add a convenience method to test that.

Change-Id: I26d77cc8f109820cca3f578c3fa3520e802dc15f
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>