Dynamic GL support in Quick1's ShaderEffectSource

Not strictly necessary but it is better not to call glEnable()
with values that are not supported by GLES2. This way the
error state will not polluted by an invalid_enum or similar.

Change-Id: I952273f1814d9f124e5c4baa6196386645202439
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>