Add new draw and loadSVG methods to the SvgRenderer

[adroitwhiz]

Resolves

A step towards resolving scratchfoundation/scratch-render#518

Proposed Changes

This PR adds two public methods and one private method to SvgRenderer:

• loadSVG, which takes in an SVG string, loads it with fromString, creates an <img> that can be rendered to a canvas, then calls the passed callback
• draw (not _draw), which synchronously draws a previously loaded SVG to the SvgRenderer.canvas
• _createSVGImage, which handles creation of the <img> itself

It also:

• Adds the loaded getter property, which returns whether or not the <img> has finished loading and can be rendered.
• Deprecates fromString and _draw. I checked scratch-gui, scratch-paint, and scratch-vm, and none of them use these methods--it's only scratch-render that uses them, and with Synchronous SVG mipmaps scratch-render#527 merged, it will stop using them.

Reason for Changes

Currently, the SvgRenderer's methods conflate:

• The asynchronous operation of loading a new SVG as an <img> element
• The synchronous operation of drawing an already loaded SVG to a <canvas> at an arbitrary scale

This can result in unpredictable behavior. For instance, the _draw method executes asynchronously the first time it is called after a new SVG has been loaded, as it has not yet created an <img> from the SVG. However, subsequent calls will be synchronous, as the <img> has been created and can be synchronously drawn to a <canvas>.

These changes make the synchronous/asynchronous split explicit with new methods that are either always synchronous or always asynchronous.

[adroitwhiz]

I did some weird Git stuff to squash commits-- the review feedback is addressed in "Reuse _cachedImage"

[adroitwhiz]

This one should stay as this._cachedImage === null to avoid creating duplicate images

[fsih]

LG! Thanks!