feat: Add clearRect before draw background.

[Fozei]

When the background color is set with a transparency parameter (such as "transparent" or when the fourth parameter of rgba is not 1), the canvas retains previously drawn content.

Reproduction steps:

1. set gridBackgroundColor to "transparent"
2. Set the grid.data array to a new array that its length shorter than it was previously.

[TonyGermaneri]

Thanks for the fix!