Add Alpha to Coverage

[JohannesDeml]

Alpha to Coverage is used for anti aliasing Cutout borders with MSAA.

See https://medium.com/@bgolus/anti-aliased-alpha-test-the-esoteric-alpha-to-coverage-8b177335ae4f

[JohannesDeml]

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It is important to use Mipmaps preserve Coverage in the texture, so it won't fade out when a lower mipmap level is used. More Information

Please note, that A2C will only with with MSAA enabled!