[Impeller] OpenGLES: Ensure frag/vert textures are bound with unique texture units.

[bdero]

The fragment shader texture bindings will smash into the texture units used for the vertex shader bindings if the vertex and fragment shaders both have textures.

Entities doesn't use any pipelines that tickle this case.

[chinmaygarde]

I am assuming this is not workflows not currently used in Flutter. But just a heads up that sampling textures from vertex stages should be behind a capability check in OpenGL ES.

[jonahwilliams]

LGTM

I'm not really familar with the capability check required for textures in vertex shaders. But
If we need to support this in general, we'll need some Vulkan specific fixes too:

• Our Vulkan barriers assume we don't read from textures in the vertex stage.
• The Vulkan render pass code assumes no texture units will be bound in the vertex stage.

These don't need to be fixed now, especially as I would prefer that we not just pessimize the barriers more 😆 😢

I think metal might just work today.

[bdero]

Metal for sure works today. And yup, good to know -- we'll definitely need this for 3D animation, so I'll probably need to fix up the barrier once I get around to smoke testing Scene or Flutter GPU stuff against Vulkan.