Now that the new 3D viewport draws to a multisample offscreen buffer, there is
no good reason anymore to create an entire multisample window and pay the
performance/memory cost for other regions that don't need it.
GL_MULTISAMPLE now only gets enabled for offscreen buffers, so we don't need
to check for it throughout the UI code anymore.
Differential Revision: https://developer.blender.org/D3062
7028a11db9