[html5] PNG file use RAW color space instead of sRGB

when using a PNG file texture, the color is fine in blender (using sRGB color space) but when exported to [html5]
the PNG textures looks washed out and much brighter as if it uses the RAW color space.

When replacing the texture by a JPG file, the colors is correctly using the sRGB color space.

Reproduce the HTML5 export with the same PNG texture and compare it with the JPG case and Blender's sRGB display. Trace where exported PNG textures receive their color-space handling; done means PNG colors match the sRGB appearance seen in Blender.

Written by the indexing model from the issue text.