Added support for PNG ROM previews.

Previews are in PNG format, must be in PNG format and must be named "rom
filename + extension".png
menu.cpp: Correctly check that the newly-selected ROM is different from the previous one to load its PNG preview. Accept previews without the extension of the file containing the ROM (.sfc, .smc, .zip), so NAME.png instead of NAME.smc.png.
Fixed black-dot issue
Merge branch 'master' of https://github.com/Nebuleon/PocketSNES