No alpha to coverage for ES3.1 and mobile Vulkan

MSAA is not applied to masked textures/materials.

Testing blocked in MAIN 4.25 @ CL 11144351 by [Link Removed]

1. In project settings add 'Quest' to "Package for Oculus Mobile devices" array
2. Enable Forward Rendering + Mobile MSAA 4x
3. Place Text Render Actor or Masked Material in scene (along with static meshes for comparison)
4. Deploy to Quest using either ES3.1 or Vulkan

Result - Masked items are not aliased