[feature] Rework render target saving to convert the image to the desired filetype

[Derpius]

Is your feature request related to a problem? Please describe.
Currently you need to copy from your RGBFFF rendering RT to an RGB888 RT pixel by pixel to save to a PNG.

Describe the solution you'd like
Either convert a clone of the RT to an equivalent 8 bit per channel format, or if stb_image_write allows writing float data to PNGs then use that.