Enhanced asset loader compatibility

This release features two major improvements to supported assets:

• Supports standard .json array format for spritesheets. This format is exported by major pixel art applications: Aseprite, Texturepacker, Piskel... Legacy .txt and .csv formats are supported too.
• Loads 24 and 32 bpp pixel art bitmaps directly, no need to convert to 8-bit indexed first.

Keep in mind that the internal rendering pipeline is still based on indexed color. But the loader can take a 24/32 bpp image, and if it has less than 255 unique colors (common for pixel art) then converts it to an equivalent 8-bit indexed on the fly. If the image has more than 255 colors it won't be loaded.

The "forest" sample has now the spritesheet in .json format and graphic assets in 24/32 bpp to showcase these new features.