Add support for HD textures when loading textures for the GUI

[Archez]

We had simple support for loading certain types of textures into ImGui directly to be able to render them for things like the item tracker/save editor. There was different logic for loading an rgba image vs a greyscale image.

However with HD textures, all images are in the rbga32 format (even greyscale images).

This makes it so we check the resource's flags to see if we should treat it as "raw" and load the texture data differently. For HD textures, the raw buffer is loaded similarly to how it is done in gfx_pp.cpp, albeit with less of the safety/size checking (I'm not sure how much of that needs to be done here).

Tested with the OoT reloaded HD texture pack and confirmed that greyscale images like the song note textures are now rendering correctly.