glamor: Learn about GL_EXT_blend_func_extended

This is apparently how GLES spells GL_ARB_blend_func_extended.
4 jobs for glamor-ext-blend-func-extended in 3 minutes and 55 seconds (queued for 1 second)
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Status Name Job ID Coverage
  Docker Image
passed debian-testing #4337401

00:00:15

 
  Build And Test
passed autotools #4337402

00:03:39

passed meson #4337403

00:01:13

passed meson-noglamor #4337404

00:01:20