replace inline asm with _fmemcpy to copy ROM font char.

replace inline asm with _fmemcpy to copy ROM font char.
add reminder that it might be faster for modexDrawChar to point
directly at ROM font char.

the reason for replacing the inline assembly is that the original
code acted without saving/restoring CPU registers that the C compiler
optimization stage assumes holds results from previous C compiler
generated code. corruptiong the optimizer's expectations is the reason
why all code using modex (and assuming things about DS and ES) is the
reason all game code was blowing up. this fix makes things stable again.