[gles] surface from canvas supports only srgb

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Description
Surfaces created from a canvas in gles only support RGBA8srgb. Would expect non-srgb to work as well

Haven't checked how prevalent this is, might happen in other configurations as well and just be a property of the gles backend.

Platform
gles / wasm / browser

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@cwfitzgerald mentioned that srgb is actually emulated for this target, so shouldn't be too hard to disable the emulation if a user selects RGBA8

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related to #1925