Change MAXSKINS from 256 to 255

!1466 (merged) increased MAXSKINS from 32 to 256, with a comment saying that it mustn't be set higher than 256 as some pieces of code use an 8-bit integer (0 to 255, or 256 different values) for skin numbers.
However, some of those pieces reserve a value of -1 (255) to mean "no skin", leaving only 0 to 254, or 255 different values, free for skins to occupy. This merge request reduces MAXSKINS to 255 so that a 256th skin won't conflict with "no skin".

For changelog purposes, this should be bundled into the "Raise the skin limit" part.

src/doomdef.h

@@ -244,12 +244,12 @@ extern char logfilename[1024];
 #define MAXPLAYERNAME 21
 #define PLAYERSMASK (MAXPLAYERS-1)
 
-// Don't make MAXSKINS higher than 256, since skin numbers are used with an
-// UINT8 in various parts of the codebase. If you do anyway, the data type
-// of those variables will have to be changed into at least an UINT16.
+// Don't make MAXSKINS higher than 255, since skin numbers are used with an UINT8 in
+// various parts of the codebase, and one number is reserved. If you do anyway,
+// the data type of those variables will have to be changed into at least an UINT16.
 // This change must affect code such as demo recording and playback,
 // and the structure of some networking packets and commands.
-#define MAXSKINS 256
+#define MAXSKINS 255
 #define MAXCHARACTERSLOTS (MAXSKINS * 3) // Should be higher than MAXSKINS.
 
 #define COLORRAMPSIZE 16