Introduce "Renderer" abstraction layer

Adds a `renderer` class for which a rendering backend must implement and
will conditionally use OpenGL in the case that `ENABLE_GL` is enabled.

src/renderer.cpp

@@ -0,0 +1,4 @@
+#include "renderer.hpp"
+
+Renderer::Renderer(GPU& gpu, const std::array<u32, regNum>& internalRegs) : gpu(gpu), regs(internalRegs) {}
+Renderer::~Renderer() {}