GPUVolume rendering was activating texture units and never
deactivating them.  The TextureUnitManager was allowing this
when it should not have been.

GenericCompoistePolyDataMapper was not releasing graphics
resources on its helper.

Added in some opengl error reporting logic that someone had added
to only the old backend.

DepthPeeling was not releasing all of its graphics resources.

Removed a couple extra includes.

Added a MakeCurrent in TextureObject when it releases its
graphics resources. Sometimes this happens when another window
is current as a side effect.