Safe texture compression (#2808)

Only a single program was used to safely compress all textures in MTG.

 * `optipng -o7 -zm1-9 -nc -strip all -clobber %%f`

The key command used is -nc, which stops color mode changes. I.e. any RGBA textures will remain RGBA textures, with none being changes to 8-bit indexed mode.

utils/optimize_textures.sh performs this compression automatically, assuming optipng is installed.

utils/optimize_textures.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+# Colors with 0 alpha need to be preserved, because opaque leaves ignore alpha.
+# For that purpose, the use of indexed colors is disabled (-nc).
+
+find -name '../*.png' -print0 | xargs -0 optipng -o7 -zm1-9 -nc -strip all -clobber