Reduce amount of canvas commands issued to the dotplot renderer

[cmdcolin]

This fixes a visual glitch I observed in the dotplot renderer. In some cases, small miniscual <1px lines were left alongside the larger diagonals

before (see very thin lines extending from larger diagonals)

after (no more very thin lines)

this also reduces the amount of canvas drawing commands issued to the renderer, which should make it faster, as number of canvas commands can be a significant portion of render time

[cmdcolin]

note that a similar optimization already exists on the linearsyntenyrendering which has to update very fast on each frame during scrolls