Clean up mountain clouds

Delete fully transparent images and associated rules

Use @V technology to reduce remaining rules to 1

Rename images to reflect new simpler layout

data/core/terrain-graphics.cfg

@@ -195,13 +195,7 @@
 {TERRAIN_BASE_RANDOM     Md                                                        hills/dry}
 {TERRAIN_BASE_RANDOM     Ms                                                        hills/snow}
 
-{OVERLAY_LF              *^Xm                                         1   clouds1  mountains/cloud1_1}
-{OVERLAY_LF              *^Xm                                         1   clouds2  mountains/cloud1_3}
-
-#33% 33% 33%
-{OVERLAY_PLF             *^Xm                                   33    1   clouds3  mountains/cloud1_2}
-{OVERLAY_PLF             *^Xm                                   50    1   clouds3  mountains/cloud2_2}
-{OVERLAY_LF              *^Xm                                         1   clouds3  mountains/cloud3_2}
+{OVERLAY_LF              *^Xm                                         1   clouds  mountains/cloud@V}
 
 {PEAKS_1x2_SW_NE         *^Xm                                   15        peaks    mountains/peak_range1}
 {PEAKS_LARGE             *^Xm                                   25        peaks    mountains/peak_large1}