Polish rendering driver refactor further

Mainly:
- Make `max_descriptors_per_pool` project setting Vulkan-specific.
- Use a common, render driver agnostic magic FourCC for shader binary data.
- Downgrade spirv_reflect to Vulkan-only dependency.
- Add a `RENDER_DRIVER_*` macro to GLSL shader code for per-driver customizations.

servers/rendering/renderer_rd/shader_rd.cpp

@@ -180,6 +180,7 @@ void ShaderRD::_build_variant_code(StringBuilder &builder, uint32_t p_variant, c
 #if defined(MACOS_ENABLED) || defined(IOS_ENABLED)
 				builder.append("#define MOLTENVK_USED\n");
 #endif
+				builder.append(String("#define RENDER_DRIVER_") + OS::get_singleton()->get_current_rendering_driver_name().to_upper() + "\n");
 			} break;
 			case StageTemplate::Chunk::TYPE_MATERIAL_UNIFORMS: {
 				builder.append(p_version->uniforms.get_data()); //uniforms (same for vertex and fragment)