Fix webgl erase. Resolves #6523

[acamposuribe]

Resolves #6523

Changes:
Changes to the erase() and noErase() functions in the webgl renderer.
In the erase() function

• Activate isErasing at the beginning
• Update _cachedBlendMode with current BlendMode (if omitted, it does not pass npm test)
• Change blendMode to constants.REMOVE mode

In the noErase() function

• Store currentBlendMode in temp
• Change blendMode back to the original (cached)
• Set cached to temp. If we don't do this, when applying the new blendmode (applyBlendMode()), the function returns null
• Apply original blendMode and set cached again to the original (to pass npm test)

This is all very confusing, but it's the only way of making it work without refactoring the whole thing.

PR Checklist

• npm run lint passes

[welcome]

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[davepagurek]

Thanks so much for fixing this and diving into the renderer internals!

[davepagurek]

@all-contributors please add @acamposuribe for bug, code

[allcontributors]

@davepagurek

I've put up a pull request to add @acamposuribe! 🎉