Create rgba_add_clamp()

For adding to a color without overflowing components.
add rgba_add_clamp() to header
High contrast cave rendering

Instead of alpha blending depth color, it adds a luminance-neutral depth color.

Also fixed the glitch at Y=64.
Merge branch 'master' of https://github.com/dtfinch/mapcrafter into dtfinch-master
Merge branch 'dtfinch-master' into cave_contrast_patch
Fixed util::trim function to work also with Windows newlines...
Added an option for new high contrast cave rendering.
Documented new cave_high_contrast option.
Adjusted style of high contrast cave render code a bit.
(for consistency)
Merge branch 'cave_contrast_patch'