Allow ShaderEffect items to be batched

A ShaderEffect might be used to apply the same effect to a number of different, small images. For example, imagine using it to "dim" a large number of disabled icons. This approach is generally quite convenient in the QML, but can have a negative impact on rendering performance.

As discussed with Gunnar, to allow these to be batched, the following changes could be implemented (all 3 are required):

• images provided to the ShaderEffect without being removed from the atlas (this has the risk of allowing a ShaderEffect to access the entire atlas), and
• QQuickShaderEffectMaterial have a smarter compare() function, and
• ShaderEffect have API to specify that it will stay in bounds (all painting will be confined to the geometry of the ShaderEffect within the Item tree)