Add option to set color for rendering nodata pixels in raster layers

[nyalldawson]

Allows raster nodata pixels to be colored in a specific (non-transparent) color. Designed to match the "Display background value" option from ArcMap.

Sponsored by SLYR

[nirvn]

This was reported a while back as an implementation gap by ESRI users migrating to QGIS (see for e.g. issue #26087).

[nirvn]

@nyalldawson , tested the branch, nodata color working with all of our raster renderers, and the UI looks nice. Good job:

[stale]

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[stale]

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