Fix internalFormat in QOpenGLTexture for ES2

Task-number: QTBUG-39856
Change-Id: I005f05c26efb4c81dd017cd4b0f16f6dac3f679e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Use RGBA8888 always in the eglfs backingstore

Do not try to be clever, this format works always.

Using RGBX8888 for windows without alpha buffer was a bad idea because
it broke renderToTexture widgets (QQuickWidget, QOpenGLWidget) which
failed to properly draw the transparent rect with this format.

Change-Id: I295d2f8c17490b59cc5e6b9d81035360da28ab3d
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Avoid calling glGetString on OpenGL 3.0+

To prevent profiling and debugging tools from generating misleading
"errors", call glGetString only for ES and GL < 3.0.

Task-number: QTBUG-35173
Change-Id: I4fe3b048fff4cda948152e2ceeb1347f2b0e4ca5
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Add missing doc file for cocoa native context

Change-Id: I11938311f648ba63bd9bee2e518c7525871cd0a3
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>