[gles] enable/disable blending per attachment only when available (on ES 3.2 or higher)

[teoxoy]

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
fixes #2987

Description

• enable/disable blending per attachment only when available (on ES 3.2 or higher)
• remove PrivateCapabilities::CAN_DISABLE_DRAW_BUFFER as it's equivalent to DownlevelFlags::INDEPENDENT_BLEND
• fix order of args given to enable_draw_buffer & disable_draw_buffer (disable_draw_buffer has draw_buffer and parameter backwards grovesNL/glow#10)

Testing
CI & @Zoxc could you try this patch? (I don't have a device with ES 3.1 or lower)

[cwfitzgerald]

I don't quite get how this works in the es 3.0/1 case where the disabling can just be removed. If we can just remove it, why do it at all

[teoxoy]

I don't quite get how this works in the es 3.0/1 case where the disabling can just be removed. If we can just remove it, why do it at all

It took me a while to see that all functions ending in draw_buffer have to be gated behind the DownlevelFlags::INDEPENDENT_BLEND and most were already. Those that were not, I removed since I think they were not necessary in the first place.

There was a call to disable blending for all targets in perform_shader_clear already, there should be no need to do it again for each individually.

As for SetDrawColorBuffers(count), we were disabling blending on each target individually if not targeting wasm (note the check should have been es 3.2 not wasm) but didn't have fallback for wasm where we would disable blending on all targets via gl.disable(glow::BLEND). I think this was unnecessary since we enable/disable blending later anyway.