Detect constant shapes (slope = 0) to optimize scanline processing & tile generation

[bourgesl]

Detect that edges are all hz or vertical :

• v lines or rectangles are "constant"
• Use 1px row and repeat it directly until new edges to compute tiles.
• If the line or height is high, it will compute only 1px row mask instead of (y2 - y1) pix * 8 (subpixels) !

That would be a huge win.

[bourgesl]

See sketch showing vertical lines or rectangles: all edges have no slope...

[bourgesl]

Optimize addLine too to avoid computations when slope ~ 0