I18N: Switch to getting categories by index instead of by string lookup

Also gets rid of the shared_ptr usage, and generally makes things nicer.

Needed for later config refactorings, good to get in early.

GPU/Vulkan/TextureCacheVulkan.cpp

@@ -511,7 +511,7 @@ void TextureCacheVulkan::BuildTexture(TexCacheEntry *const entry) {
 		// TODO: We should stall the GPU here and wipe things out of memory.
 		// As is, it will almost definitely fail the second time, but next frame it may recover.
 
-		auto err = GetI18NCategory("Error");
+		auto err = GetI18NCategory(I18NCat::ERRORS);
 		if (plan.scaleFactor > 1) {
 			System_NotifyUserMessage(err->T("Warning: Video memory FULL, reducing upscaling and switching to slow caching mode"), 2.0f);
 		} else {