Revisit SVG costume subpixel positioninging for non-integer viewboxes

[towerofnix]

Issue corresponds to this code to-do:

leopard/src/renderer/VectorSkin.js

Lines 74 to 76 in 581bf25

	// TODO: handle proper subpixel positioning when SVG viewbox has non-integer coordinates
	// This will require rethinking costume + project loading probably
	_createMipmap(mipLevel) {

Follow-up on #119 (comment):

@adroitwhiz wrote:
I wrote up a summary of this a while back, over in scratchfoundation/scratch-render#550. There's an associated PR that fixes it in scratchfoundation/scratch-render#594.

There's a good deal of discussion in those links. This is a bigger issue to address later which, as the to-do notes, may require or be easier to approach with costume/project preloading (#133) instead of generating costumes/skins/etc "on demand" (during execution).