LibSoftGPU: Implement shader processor for SoftGPU ISA

This adds a shader processor that executes our ISA when a fragment
shader is currently bound to the device.

Userland/Libraries/LibSoftGPU/CMakeLists.txt

@@ -3,6 +3,7 @@ set(SOURCES
     Device.cpp
     Image.cpp
     PixelConverter.cpp
+    ShaderProcessor.cpp
     Sampler.cpp
     Shader.cpp
 )