LibGL+LibSoftGPU: Move rendering related code to LibSoftGPU library

This introduces a new library, LibSoftGPU, that incorporates all
rendering related features that formerly resided within LibGL itself.

Going forward we will make both libraries completely independent from
each other allowing LibGL to load different, possibly accelerated,
rendering backends.

Userland/Libraries/LibGL/CMakeLists.txt

@@ -1,9 +1,4 @@
 set(SOURCES
-    Tex/NameAllocator.cpp
-    Tex/Sampler2D.cpp
-    Tex/Texture2D.cpp
-    Tex/TextureUnit.cpp
-    Clipper.cpp
     GLBlend.cpp
     GLColor.cpp
     GLContext.cpp
@@ -18,9 +13,11 @@ set(SOURCES
     GLVert.cpp
     GLVertexArrays.cpp
     SoftwareGLContext.cpp
-    SoftwareRasterizer.cpp
-    DepthBuffer.cpp
+    Tex/NameAllocator.cpp
+    Tex/Sampler2D.cpp
+    Tex/Texture2D.cpp
+    Tex/TextureUnit.cpp
 )
 
 serenity_lib(LibGL gl)
-target_link_libraries(LibGL LibM LibCore LibGfx)
+target_link_libraries(LibGL LibM LibCore LibGfx LibSoftGPU)