Use luminance multiplier for sky background when using mobile renderer with HDR2D

[clayjohn]

Fixes: #113597

The sky_luminance_multiplier is fixed at 0.5 since the radiance map will always use an LDR format. When rendering to the viewport directly, we should use inverse_luminance_multiplier which changes between 1.0 and 0.5 depending on if HDR2D is enabled or not.

The rest of the sky update functions render to LDR textures always, so they need to use the sky_luminance_multiplier

[akien-mga]

Thanks!