[Impeller] split downsample shader into GLES variant.

[jonahwilliams]

This fixes a crash reported on a moto g4 where the on device shader compiler fails to process the ifdef.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[auto-submit]

auto label is removed for flutter/engine/56376, due to - The status or check suite Mac mac_unopt has failed. Please fix the issues identified (or deflake) before re-applying this label.