Force alpha to 1 in tonemap pass in RD renderers

[clayjohn]

Fixes: #83131 for X11
Fixes: #61969

Also only clear to alpha 0 when using transparent background

This change isn't needed in the compatibility renderer as it already correctly checks for transparent_bg when setting the clear color and it avoids writing to alpha when not needed.

Setting to draft for now as I have only tested with the MRPs from the two projects. Its possible that other effects rely on this previous erroneous behaviour. So we need to test carefully.

[akien-mga]

I confirm that this fixes both linked bug reports for me on Linux X11.

Tested with these three Vulkan devices, both Mobile and Forward+:

Vulkan API 1.3.246 - Forward+ - Using Vulkan Device #0: Intel - Intel(R) HD Graphics 630 (KBL GT2)
Vulkan API 1.3.246 - Forward+ - Using Vulkan Device #0: AMD - AMD Radeon RX Vega M GL Graphics (RADV VEGAM)
Vulkan API 1.3.246 - Forward+ - Using Vulkan Device #0: Unknown - llvmpipe (LLVM 15.0.6, 256 bits)