fix: Create CSS vars for SVG patterns.

[johnnesky]

The basics

• I validated my changes

The details

Resolves

Fixes #8272

We'll need a related PR in blockly-samples that adapts the screenshot plugin to be aware of CSS vars.

Proposed Changes

Adds CSS variables scoped to the injection div referencing the SVG pattern ids so that CSS can use these consistent variable names instead of the random ids that were previously assigned to SVG attributes.

Reason for Changes

So that renderer CSS can customize and override how SVG patterns are used.

Test Coverage

I did not create any new tests, but all existing tests pass and I manually tested that the filters appear to be rendering correctly.

Documentation

N/A

Additional Information

N/A

[gonfunko]

I think this also fixes #1403?

[johnnesky]

I think this also fixes #1403?

Depends on what they wanted to do to the disabled pattern. I wouldn't close that yet without putting some thought into it.