Marcel van Kervinck

First step in https://github.com/kervinck/gigatron-rom/commit/e610d8a9f3e55c22f66c3f68dfbf668e66749728

Low priority

Low priority

Two ideas to reduce the number of thunk functions in rt.py: 11. Move the start of the pixel lines to offset 0x60: ```videoTable[1] = 0x60;``` No need to let code...

13. Use XORW instead of SUBW before the == and != operators. (Sometimes the compiler even juggles the order of SUBW operands...)

14. MULI2(CNSTI2(1), ... Simplify. Same for CNSTI2(0) and MULU2, DIVXX etc

15. Remove 'rv' from rt.py, use 'ha' instead for return values

16. More aggressive purging of unused library functions. For example: ``` int main(void) { return 0; } ``` Still gives an .gt1 file of more than 4 KB in size....

17. LCC inserts explicit return values in places where "don't care" will work. This makes some code larger than needed. See this comment for an example: https://github.com/kervinck/gigatron-rom/issues/76#issuecomment-497897449

18. Some C11 code crept into src/gt1.md. Workaround solved with #97. It's would be nicer to make it all ANSI C compliant (aka C89).