[Impeller] GLES backend needs method to "Store/Load" multisample stencil or depth buffer.

[jonahwilliams]

For Impeller on the OpenGLES backend, we're going to use https://registry.khronos.org/OpenGL/extensions/EXT/EXT_multisampled_render_to_texture.txt . While this extension automagically resolves the 4x multisample texture to a 1x resolve texture, I don't believe it similarly makes the stencil attachment availble. Instead, it likely discards the stencil and depth buffer.

This should be visible by running an application with a backdrop filter - we depend on storing the stencil buffer for correct rendering.

In order to "store" the stencil buffer, we may need to manually blit or draw the stencil into another texture. I don't have a good handle on whether or not this is easily possible. If not, then we'll need to implement #137448 which removes the need to store the stencil altogether (better long term but will be a heavy lift).

@matanlurey

[jonahwilliams]

It sounds like we may not be able to do this in general. From https://registry.khronos.org/OpenGL/extensions/EXT/EXT_multisampled_render_to_texture2.txt

1. How should downsampling depth/stencil surfaces be handled?

Proposed: Ideally, when using this extension, depth/stencil attachments should
always be discarded/invalidated by the application *before* the rendering is
submitted.  If the application fails to do so, the implementation is not required
to preserve the contents of those attachments.  A depth/stencil resolve is
equivalent to InvalidateFramebuffer for those attachments.

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