Add possibility to mirror ShaderEffectSource generated textures

Add possibility to mirror ShaderEffectSource generated textures

Using textures generated by ShaderEffectSource items (or Item.layer)
with custom OpenGL code was non-intuitive due to mismatching coordinate
systems, so added a possibility to control the generated texture
orientation.

[ChangeLog][QtQuick][ShaderEffectSource] Added possibility to mirror
generated OpenGL texture.

Change-Id: I7c03d8b6fbfc43d69812c15d244200fb8e7c7bb9
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>