EMSCRIPTEN: make Xemu somewhat usable when running inside a web browser

[lgblgblgb]

Xemu/C65 for example works fine inside a web browser. Xemu/Mega65 should work as well. Interestingly it compiles, but throws a Javascript Exception, which indeed, should not happen at all.

PROGRESS

• MEGA65 emulation works again to any extent [DONE]
• Check other emulators [TODO]
• New better/more compact compressed disk image support [DONE]
• Make at least OSD menu work (seems to be a problem here that OSD menu is not in the main loop which does not work for empscriten) [TODO]
• Custom emulation page, with abilities to "upload" D81 disk images, "parameterize", etc [TODO]

[lgblgblgb]

NOTE: though web build of Xemu/Mega65 sounds not so sane and usable, actually it can be important to attract people for a fast look and feel on Mega65 (later even from Mega65 website!) which can cause more people to feel the need to try it out more ;)

[lgblgblgb]

At the current stage, maybe no emscripten complication works at all, and not so much a priority at the middle of more serious tasks / rewritings. Let's deal with it later, probably even at Xemu level, not just with MEGA65 target in Xemu.

[lgblgblgb]

Note to myself: it's possible that it's caused by the large memory needs of the MEGA65 emulator, thus some Emscripten magic is needed to provide more memory, or to "allow to grow" option, or something like that. Though, currently, this issue is suspended, as with the VIC-IV merge and other things, Emscripten buildability would break anyway. Also it will be questionable if Emscripten build make sense, as more accurate emulation (like VIC-IV improvements) causes much higher resource usage for emulation, which may be not so much a realistic scenario inside a browser any more.

[lgblgblgb]

Other comment: SDL_HINT_EMSCRIPTEN_ASYNCIFY should be considered as general.