This fixes all cases where the color profile was broken, and
libpng warns about. It also makes almost all textures indexed
instead of RGB where possible (textures that don't have
semi-transparent pixels).
'jungleleaves simple' had completely different leaf colours to
'jungleleaves', now matched.
'leaves simple' had dark green instead of black colour for transparent
pixels (the 'leaves' texture uses black), normally these pixels would
not be rendered as opaque colours but bush nodes now use this texture
and will be rendering it as opaque. The dark green pixels resulted in
a texture lacking in contrast.
