Fix WebGL text alignment + add tests

[davepagurek]

Resolves #7480

Changes:

• WebGL's _renderText always draws text left-aligned, unlike 2D mode, which has native canvas text alignment to fall back on, so it has a custom implementation of text alignment. This wasn't correctly accounting for all combinations of alignments / provided bounding boxes, so this has since been updated.
• WebGL's textBounds were broken previously because they assume 2D mode alignment, but were calling internal text alignment methods that are overridden in WebGL mode. They now explicitly call the 2D mode methods, although this should probably be refactored more in the future.
• There were no text alignment tests for WebGL previously, so I adapted the existing tests to run for both 2D and WebGL mode. The screenshots for 2D mode were updated too because I had to pick a font that WebGL mode can also use rather than relying on the canvas default font.

Screenshots of the change:

WebGL text, correctly centered in a box:

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated