Texture decoding can overflow the thread stack

[Zaba]

texture.cpp:1759 (as of b728399) allocates the texture decoder on the stack, which may cause it to overflow. This realistically happens on OS X with the default stack size. Unfortunately, SDL2 doesn't expose an API to change the thread stack size. Allocating the decoder on the heap alleviates the issue.

[graphitemaster]

Fixed in 5fa9ff8. Thanks