LibGL+LibGPU+LibSoftGPU: Implement and expose glClipPlane

This commit implements glClipPlane and its supporting calls, backed
by new support for user-defined clip planes in the software GPU clipper.

This fixes some visual bugs seen in the Quake III port, in which mirrors
would only reflect correctly from close distances.

Userland/Libraries/LibGL/CMakeLists.txt

@@ -1,10 +1,11 @@
 set(SOURCES
+    ClipPlanes.cpp
     ContextParameter.cpp
     GLAPI.cpp
     GLContext.cpp
-    Matrix.cpp
     Lighting.cpp
     Lists.cpp
+    Matrix.cpp
     Stencil.cpp
     Tex/NameAllocator.cpp
     Tex/Texture2D.cpp