Make canvas white; fake Earl Brown scores; set line width dynamically; fix bugs.

[mepc36]

More prep work in anticipating of extending script functionality to non-Mondrian works of art.

There are 3 types of changes in this P.R., maintainability changes, feature changes, and bug fixes:

Feature Changes:

• Make canvas background white, not transparent (while it appears white on index.html, that's only because it's the default styling; downloading the image will show weird, transparent spaces where there is no RYB color.)
• Generate .png files that imitate the work of artists besides Piet Mondrian, like Earle Brown and his aleatoric music scores (as in his work "December 1952")
• Make line widths differ dynamically (a major feature of Mondrian's work, previously absent from the abilities of this mondrian-generator repo.)

Maintainability Changes:

• Removed comments that are now redundant, in view of code changes (like var renames & func modularizations);
• Divided functions into smaller, more re-usable parts;
• Made var names more specific, particularly to underline the evolution of the canvas' drawn lines from internal points, to external points, and, finally, to 2-dimensional realizations.
• Modularize tag into its own external .css file.

Bug Fixes:

• Fixed bug in use of .getRandomInt that would generate erroneous indexes for arrays (was accidentally passing in array.length, not array.length - 1.)

N.B.: There is one outstanding bug — the code here will sometimes generate undefined values. If xPtr or yPtr is the last index in an array, then xLineStarts[xPtr + 1] will obviously not exist. However, this bug is not surfacing in the actual .png images themselves anywhere, so I'll leave it aside for now.