Allow 16-bit PNG depth maps for backgrounds

[p0nce]

Currently because the depth map is 8-bit when loaded as background, you can't make slow ramps without artifacts.

For example here is the render with a depth map that contains slow ramps (modified Panagment graphics)
16-bit depth

instead of

8-bit depth

This limitation doesn't affect runtime primitives that draw into depth properly, it's only image resources.
A former workaround was to increase precision by using a color like (255, 254, 253), when reduced to 16-bit grey it would increase precision.

• Support loading 16-bit PNG as depth map
• Constraint: legacy depth maps using 8-bit RGB PNG shoud keep working with only minor visual change.

[p0nce]

Visual effect of former loading vs directly loading as L16 is pretty neutral (not really better), yet pretty visible.

[p0nce]

Checked in, will be available in v12.0.9

8-bit depth maps load as before.
16-bit depth maps are built more directly. So it is not a breaking change.

Using a 16-bit greyscale depth map (you can set in GIMP) changes the following:

• saves 1.8mb of RAM in Panagement, that is 1.5% of its total memory consumtion.
• not really seen a change in load time

When converting the map in GIMP be sure to first augment bitdepth before converting to greyscale.