Allow source assets to be stored as their raw data format (PNG, FX)

Summary: This implements support for loading source assets as their raw format (PNG and FX files).  Thus this means the source assets don't need to be stored as .asset files for simple things like textures, where the only relevant information is the texture data itself.

Test Plan: Removed the old texture .asset files in Tychaia and moved the .png files in their place and everything worked.

Reviewers: Sabre070

Reviewed By: Sabre070

Differential Revision: https://code.redpointsoftware.com.au/D135