Fix the detection of alpha-channel in textures as created by IMG_Load - Take two

This is the reopening of https://github.com/colobot/colobot/pull/243 as I can't reopen it.

It was closed after being merged but the changes were quickly reverted with the https://github.com/colobot/colobot/commit/70af33e45d286bc6f62a392d0b967aafdc7548e4 "Looks like it doesn't work on all platforms" argument.

Surprised by that statement, I compiled colobot at the tip of this branch on all three available platforms; Linux (Debian Jessie), Mac OS X and Windows (using a stable MXE branch) and tested it natively (including booting a Windows 7 machine) on all of them; textures' loading works and the game successfully runs on all these platforms.

A better description of the problem would help me provide fixes (rather than just reverting the fix with such a concise explanation).

Thanks for considering, cheers,

OdyX