SHR PLOT enhancements

[bobbimanners]

Applecorn does not support all of the possible values of k for PLOT k,x,y statements.

Implemented:

• k=0..7
• k=64-71

Not implemented:

• k=8-15 Should draw line with last point omitted
• k=16-23 Dotted line
• k=24-31 Dotted with last point omitted
• k=72-79 Filling mode
• k=80-87 Triangle filling

[bobbimanners]

I really should implement k=80-87 Triangle filling

[bobbimanners]

Triangle filling:

• Store two previous points visited, not one as at present.
• Triangle is made up of two previously visited points, plus the current plot point.
• Sort the three vertices in y-coordinate order (top to bottom)
• Divide the triangle into two halves, separated by a horizontal line extending from the middle y-coordinate
• For each of the two half-triangles:
  • For each row of the half-triangle:
    • Compute coord of LHS using something Bresenham-ish
    • Compute coord of RHS using something Bresenham-ish
    • (Efficiently) plot a line from LHS to RHS in current row, using current GCOL action (and, or, xor, whatever)