libgui|OpenGL: Detect NVIDIA CSAA support, use it if available

IssueID #1707
libgui|OpenGL: Correct exported name of glRenderbufferStorageMultisampleCoverageNV
Merge branch 'master' of github.com:skyjake/Doomsday-Engine into ui-alerts

Conflicts:
	doomsday/libgui/include/de/gui/glentrypoints.h
	doomsday/libgui/src/glentrypoints.cpp
	doomsday/libgui/src/gltarget.cpp