QOpenGLTexture: test for extensions when checking features

Don't use only the GL version, as vendors expose many many extensions
on viable hardware.

For instance, I have a NVIDIA G210 which supports up to GL3.3, but
which features immutable storage, immutable multisampled storage,
texture buffers and ranges, stencil texturing, and cubemap arrays.

Change-Id: Ie6023ee854b679737fca982578cb2093e10d083f
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>