Alpha Update 4

Another update focused on Race-mode:

* PNG maps: dramatically reduced memory usage, 8-bit images are not expanded to full color anymore, upscaling is done in the shader. E.g. Pizza map went down from 1.9GB to 140MB in memory;
* Added zoom, helps to navigate large racing maps;
* Enabled AI in racing mode (only local games at the moment);
* Fixed mines not being affected by explosions;
* Fixed jumps mid-air;
* Fixed incorrect orientation of dynamite pack;
* Fixed turn/arrow indicator appearing when no input is taken;