Add get/set pixel routines

[MaxGraey]

Currently, only for AssemblyScript. But I can also add for everyone else.
@aduros wdyt?

[aduros]

I'm feeling like we should keep default API stubs very minimal. Things like pixel drawing, polygon drawing, etc. would be better to go in 3rd party libraries that can be maintained by other users as they see fit.

[MaxGraey]

set / get pixel is quite basic and useful operation which is pretty tricky in wasm4 due to packing. And users may easily do it in suboptimal way. See example: f128495#diff-222620e907510b8af2a9e5b17a92c840a445fefa24f760b26babd640fa7eb6f8R745

[haviital]

I'm feeling like we should keep default API stubs very minimal. Things like pixel drawing, polygon drawing, etc. would be better to go in 3rd party libraries that can be maintained by other users as they see fit.

Would that 3rd party library eat the fixed 64 kb memory area reserved for the game etc.?