Add string-based renderer backend configuration

Rather than using integer-indices, just use plaintext case-insensitive
names and leave the actual enum indexes as an implementation detail.

src/renderer.cpp

@@ -1,4 +1,24 @@
 #include "renderer.hpp"
 
+std::optional<RendererType> fromString(std::string inString) {
+	// case-insensitive
+	std::transform(inString.begin(), inString.end(), inString.begin(), [](unsigned char c) { return std::tolower(c); });
+
+	if (inString == "null")
+		return RendererType::Null;
+	else if (inString == "opengl")
+		return RendererType::OpenGL;
+
+	return std::nullopt;
+}
+
+const char* toString(RendererType rendererType) {
+	switch (rendererType) {
+		case RendererType::Null: return "null";
+		case RendererType::OpenGL: return "opengl";
+		default: return "Invalid";
+	}
+}
+
 Renderer::Renderer(GPU& gpu, const std::array<u32, regNum>& internalRegs) : gpu(gpu), regs(internalRegs) {}
 Renderer::~Renderer() {}