Fix support for depth/stencil textures in QOpenGLTexture

When allocating (mutable) storage for a texture, OpenGL mandates that
the client-side format and pixel type "match" the internal format.
That needs to happen even if we are not actually uploading anything
(because we're passing NULL as the data parameter and no PBO is bound).

This means that we need to pick a compatible format/type and not just
pass GL_RGBA / GL_UNSIGNED_INT. In turn, it implies adding new
enum values to the various format/type enums.

Change-Id: If40c63b1d44764b3be131dd1b41d13983a19ae45
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>