Texture conversion fails with grayscale PNGs

[historic_bruno]

On some systems like mine, the conversion from 8-bit Grayscale PNG to 32-bit RGBA (tex_transform_to) silently fails, leading to a crash in NVTT's InputOptions::setMipmapData which expects 32-bit RGBA data. For now the workaround is to not add grayscale PNGs to the game's data :) The solution probably involves extending plain_transform to correctly handle grayscale data.

[historic_bruno]

Example of texture that causes the crash: art/textures/skins/gaia/decal_quarry_savanna_a_spec.png​

[ben]

In 12559:

Replaces grayscale decal texture with RGB, refs #1640

[ben]

In 12560:

Displays error when converting a grayscale texture instead of possibly crashing, refs #1640

[historic_bruno]

Re #3082, Deiz brought up a good point: the libpng API can be used to do some transformations. Here is an article describing a few: ​http://www.vias.org/pngguide/chapter13_08.html

We could use the GDK Pixbuf library (LGPL) as a reference: ​https://www.imagemagick.org/subversion/librsvg/trunk/gdk-pixbuf/gdk-pixbuf/io-png.c

[ben]

In 16407:

Reverts action-heal cursor from r16386, it was modified to indexed color not currently supported by the engine, fixes #3082, refs #1640. Reapplies the transparent color removal, refs #2823.
Deletes unused test-old cursor

[historic_bruno]

Going to use this ticket as a placeholder for the other PNG formats we could support. The changes required seem relatively simple, we just need time to test the changes (not days before a release).

[ben]

In 16439:

Improves handling of PNG textures based on GdkPixbuf's setup_png_transformations. The PNG decoder should now support grayscale, RGB or indexed color; bit depth up to 16; and interlacing. Fixes #1640, refs #2823, #3082

[ben]

In 16449:

Fixes grayscale PNG handling, there is no need to convert them all to RGB and r16439 broke special font texture handling. Instead, only grayscale with alpha PNGs will be converted to RGBA (they fail to load otherwise).
Removes warning about converting grayscale textures, refs #1640.