Allow underscores in guifont PostScript names

MacVim guifont's can be specified by either full PostScript names or
family names. Since family names can contain spaces, MacVim allows using
underscores to serve as substitute to make it easier for the user to
type, and this is similar to how Win32 gVim works.

However, some fonts use underscores in their full PostScript name. For
example, Cascadia Code has names like "CascadiaCode-Regular_Light". The
previous substitution code always replaced underscores with spaces which
was wrong. Fix it to only do the substitution if the font could not be
loaded. Also, fix documentation to be clearer in what MacVim expects as
guifont's input.

In retrospect, implicit substitution like this was probably a bad idea
when the user could just escape with spaces, but given this has been the
behavior for a long time we should just keep it to avoid breaking
backwards compatibility.

Fix #1208

Author: ychin

runtime/doc/gui.txt

@@ -1135,10 +1135,18 @@ That's all.  XLFDs are not used.  For Chinese this is reported to work well: >
 
 MacVim							*macvim-guifont*
 
-For MacVim you can use something like this: >
-    :set guifont=Menlo:h10
-Fonts with spaces are set like this: >
+In MacVim, you can specify fonts by either the full PostScript name or the
+family name (both can be found in the Font Book app), followed by an optional
+font size (which defaults to `h11`).  Font names can also be completed in the
+cmdline using |complete-set-option|.
+
+Basic example of setting the font to Menlo with default size: >
+    :set guifont=Menlo
+To set a font by its family name which contains spaces: >
     :set guifont=DejaVu\ Sans\ Mono:h13
+When specifying family names with spaces, you can also use underscores
+instead: >
+    :set guifont=DejaVu_Sans_Mono:h13
 To use bold/italic fonts, use the fully specified PostScript name of the
 font, like so: >
     :set guifont=Menlo-Bold:h13

src/MacVim/gui_macvim.m

@@ -1196,12 +1196,6 @@
         parseFailed = YES;
     }
 
-    if (!parseFailed) {
-        // Replace underscores with spaces.
-        fontName = [[fontName componentsSeparatedByString:@"_"]
-                                 componentsJoinedByString:@" "];
-    }
-
     const BOOL isSystemFont = [fontName hasPrefix:MMSystemFontAlias];
     if (isSystemFont) {
         if (fontName.length > MMSystemFontAlias.length) {
@@ -1228,6 +1222,16 @@
                 || isSystemFont
                 || [NSFont fontWithName:fontName size:size])
             return [[NSString alloc] initWithFormat:@"%@:h%d", fontName, size];
+
+        // If font loading failed, try to replace underscores with spaces for
+        // user convenience. This really only works if the name is a family
+        // name because PostScript names should not have spaces.
+        if ([fontName rangeOfString:@"_"].location != NSNotFound) {
+            fontName = [fontName stringByReplacingOccurrencesOfString:@"_" withString:@" "];
+            if ([NSFont fontWithName:fontName size:size]) {
+                return [[NSString alloc] initWithFormat:@"%@:h%d", fontName, size];
+            }
+        }
     }
 
     return NOFONT;