LibGL+LibSoftGPU: Add GPU side shader infrastructure

This adds a shader class to LibSoftGPU and makes use of it when linking
GLSL program in LibGL. Also adds actual rendering code to the shader
tests.

Userland/Libraries/LibSoftGPU/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     Image.cpp
     PixelConverter.cpp
     Sampler.cpp
+    Shader.cpp
 )
 
 add_compile_options(-Wno-psabi)