Cleanup some use of depth textures in OpenGL2

VTK classes to Blits between depth buffers which can be problematic as blits between different formats is an error.

As VTK mappers and passes have to often work inside of an unknown FBO add some methods to help create depth textures that are compatible with the currently bound depth format.

There are a number of render pasees that have a DepthFormat ivar that should also be updated to default to creating a compatible format to be safe. This is especially true though of classes that perform depth blits.