Refactoring code to set color/opacity/visibility for blocks.

This commit refactors some of the code added in recent weeks to add ability for
users to set display paramaters for blocks within a composite dataset. This
commit additionally adds support for changing block opacity.

Blocks colors/opacities can also be overridden when using scalar coloring.
The only issue is when depth-peeling gets used. In that case, point-data coloring
with texture (i.e. using interpolate scalars before mapping) results in color
bleeds. We need to dig into the depth peeling code to address that issue.

Change-Id: I2c86912b45bc0516dc4dc726df9448e0569c8312