[WIP] Add cgif_rgb API

[dloebl]

Add public cgif_rgb API with the following features:

• Allows passing RGB(A) image data to cgif.
• cgif takes care of quantization/dithering and creates the color palette(s).

ToDo:

• Reuse tree mode (API)
• Make it possible to provide a partial frame
• Enable/disable dithering config switch
• Interlaced write support
• add global palette mode (reuse global palette until error is too large)
• interpalette-maxerror config (reuse global palette for subsequent frames if below threshold)
• support local palettes
• reuse palette mode
• Switch from index to raw API
• Quantization effort configuration (add parameter, but ignore for now): Check time for color quant and lookup time
• target bit depth (see quantize_and_dither())
• interframe-maxerror config (pixels shine through)
• cleanup/refactor cgif_rgb module
• add documentation to Wiki/README
• add examples
• add coverage tests for cgif_rgb module