Do not resolve core functions on GLES in texture helper

As the spec for eglGetProcAddress says, some implementations may not return
function pointers for core functions. Similarly to how we cannot get
OpenGL 1.0/1.1 functions with WGL for example.

To make sure QOpenGLTexture does not just crash with such implementations,
we simply use the statically exported functions in -opengl es2 builds.

Change-Id: I213bfcc21e58888b17e0ebcd0a26f26f77517e40
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>