Update terrain dialog and some other tweaks/fixes

- Fix documentation of how to format map graphics, to match the logic in the code
- Fix display of map graphics both on the automap and in dialogs
- Add Choose button to select the map graphic
- Fix the vanishing text bug again (this time it only triggered for fields with a single character in them)

src/dialogxml/field.cpp

@@ -201,7 +201,7 @@ void cTextField::draw(){
 	size_t ip_offset = 0;
 	hilite_t hilite = {insertionPoint, selectionPoint};
 	if(selectionPoint < insertionPoint) std::swap(hilite.first,hilite.second);
-	if(haveFocus && contents.length() > 1 && changeMade) {
+	if(haveFocus && contents.length() >= 1 && changeMade) {
 		text_rect = frame;
 		text_rect.inset(2,2);
 		// Determine which line the insertion and selection points are on

src/dialogxml/pict.cpp

@@ -122,9 +122,9 @@ void cPict::setPict(pic_num_t num, ePicType type){
 			picNum -= 10000;
 			picType += PIC_PARTY;
 		} else {
+			picType += PIC_CUSTOM;
 			if(picType != PIC_CUSTOM_TER_MAP)
 				picNum %= 1000;
-			picType += PIC_CUSTOM;
 		}
 	}
 	recalcRect();