Investigate feasibility of supporting HDR on WebAssembly

Investigate if Qt's HDR support can be made to work on Qt for WebAssembly.

Dependencies

• Qt RHI support for HDR for the OpenGL ES (3) backend.
• Native browser support for HDR WebGL canvas. Note that this is distinct from image and video HDR support. At the time of writing support for HDR canvas is limited.

Resources

Chrome HDR test cases: https://ccameron-chromium.github.io/webgl-examples/canvas-hdr.html

Enabling HDR Canvas  in Chrome

In chrome://flags, set the following:

• Experimental Web Platform features: Enabled
• Out-of-process 2D canvas rasterization: Enabled

Native WebGL (canvas) prototype
https://git.qt.io/mosorvig/qt-webassembly-development/-/blob/d74b5742ec11b4535aa090e8b46225c141cd7089/hdr/html/hdr.html

Setting the drawingBufferColorSpace on the GL context to either 'rec2100-hlg' or 'rec2100-pg' is sufficient (and neccesary) to get HDR output  in Chrome.