[Gles] Fix clearing depth and stencil at the same time

[expenses]

Description

Currently in the gles backend, if a depth+stencil buffer with a format like TextureFormat::Depth24FloatPlusStencil8 is attached to a render pass with both the depth and stencil set to clear, only the depth is cleared. I'm not sure if this happens with native gl, but it certainly does with webgl.

Testing

This fixed a problem I had in my project. Here's a before and after of using a stencil buffer for a mirror:

[cwfitzgerald]

Looks good, kinda weird we need it, but this is also an optimization, so totally fine with me

[expenses]

kinda weird we need it

Wait.... Could it be that the problem is actually that the depth buffer doesn't get cleared in the area under the stencil, because we're still stencil testing or something? I need to test what happens if we clear the stencil buffer first. Regardless though, this PR is still good because we shouldn't have to think about this stuff

[cwfitzgerald]

That's very plausible, not sure how stencil interacts with clears on GL

[expenses]

Wait.... Could it be that the problem is actually that the depth buffer doesn't get cleared in the area under the stencil, because we're still stencil testing or something? I need to test what happens if we clear the stencil buffer first. Regardless though, this PR is still good because we shouldn't have to think about this stuff

Nope, switching the two around doesn't seem to make a difference. Odd. Oh well, I've added some documentation

[cwfitzgerald]

Thanks!