bgfx: bgfx native formats colour channels don't match bimg::imageConvert

Description It looks like when fixing inconsistencies between renderers in #2029 and #1932, the channels of RGBA4 and RGB5A1 have the red and blue channels reversed.

The result of having this discrepancy is that platforms where this format is emulated will have a different colour ordering than those that natively support them.

I modified an example to draw different RGBA formats to better visualize this. First row has the native formats that bgfx uses and below are the same formats but they’ve been converted to RGBA8 using bimg::imageConvert.

I also tried out RGB10A2 because I suspected that it also suffers from inconsistencies. It seems to only have R&B inverted on Vulkan

To Reproduce

1. Checkout my branch git clone https://github.com/bwrsandman/bgfx.git -b pixelformats-example
2. Compile the 47-pixelformats example
3. Run the 47-pixelformats example with different renderers
4. See mismatch

Expected behavior The top row and bottom row should match for different formats. The color wheel starts red at degree 0, green at -60 degrees and blue at 60. Alpha interpolates between 1 in the centre and 0 at the edge.

Screenshots

Vulkan on Linux

OpenGL on Linux

Vulkan on Windows

OpenGL on Windows

Direct3D9 on Windows

Direct3D11 on Windows

Direct3D12 on Windows

Metal on OSX

WebGPU

Additional context

For the Metal on OSX, I removed two lines that force software conversion in order to show the native interpretation.