Fix loading grayscale heightmaps for RM maps.

rP2113 introduced code to read Heightmap images into RM terrain data. However, the original diff contained a bug where it read Out-of-bounds array data for grayscale images. This bug was hidden by another issue until D1816 / rP22892, which changed the code and made the OOB read actually relevant. The effect was twofold:

• The height chosen was not the max of the 3 color channels, but the max of the 3 neighboring pixel (thus slightly lowering the quality of generated maps)
• The height for the bottom-right coordinates were random memory values, thus garbage.

This random height ended up resulting in non-deterministic map generation, which was reported on Ngorongoro.

The cause of this silent failure is that the transformation to BGRA is not applied for grayscale images, as the alpha transformation is processed first and exits. This is not per-se buggy, so it is not changed here.
The fixed behaviour is specialized for the common grayscale case, and retains the max-of-3-color-channel intended behaviour otherwise.

Fixes #6261.

Reported by: Feldfeld

Differential Revision: https://code.wildfiregames.com/D4203