Remove Rendering/FreeTypeOpenGL

This branch removes the FreeTypeOpenGL module. This module only provided an out-of-date implementation of vtkTextMapper and is causing issues on the new OpenGL2 backend.

A test using the older mapper exposed a bug when the new implementation was used. In short, the vertical centered justification didn't always align to the center of the bounding box in the case that descenders weren't fully utilized. This required a change to the text rendering backend to always account for descenders during alignment.

This change invalidated many baselines by adding a slight vertical shift to some text, which in turn exposed some long-standing bugs; some tests created bad images that were only slightly off and still passed the image comparisons. Hence all of the seemingly unrelated bugfixes -- these were needed before I could update the baselines with the new text layouts.