LibGL: Rename `GLMat.cpp` to `GLMatrix.cpp`

Userland/Libraries/LibGL/CMakeLists.txt

@@ -7,7 +7,7 @@ set(SOURCES
     GLLights.cpp
     GLLists.cpp
     GLMap.cpp
-    GLMat.cpp
+    GLMatrix.cpp
     GLStencil.cpp
     GLTexture.cpp
     GLUtils.cpp