Print filenames in irrlicht png warnings

[Desour]

Warnings like these are useless, because you don't know which file causes it:
WARNING[Main]: Irrlicht: PNG warning: iCCP: known incorrect sRGB profile
With PR, it looks like this:
WARNING[Main]: Irrlicht: PNG warning for abriglass_clearglass.png: iCCP: known incorrect sRGB profile

(libpng docs: http://www.libpng.org/pub/png/libpng-manual.txt)

The change in CNullDriver::createImageFromFile is needed because the jpg loader complains with an error if the file doesn't have the jpg signature.
isALoadableFileFormat only has false negatives in the case of "very old tgas", according to tga loader code comment.
Do we support those?

To do

This PR is a Ready for Review.

How to test

• Add mod with a texture that causes a warning, e.g. this old version of abriglass:
  https://github.com/mt-mods/abriglass/blob/2e8968dc0f72952c9d8ace1fed397f5c4e985168/textures/abriglass_clearglass.png
• Start the game.
• Look into warning log.

[sfan5]

lgtm