The Windows-backend based GlyphCache is OpenGL specific, so
reflect that by renaming it.

Change-Id: I1034bfde14792f0b6a807f8e938742556a31fcfb
Reviewed-on: https://gerrit.libreoffice.org/61452
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>