Avoid loading rgb list for color names used by vim

Some colors are used by name in the default colorschemes.  Duplicate these in
their own table, to avoid loading entire rgb.txt list unless it's really
necessary.

plugin/CSApprox.vim

@@ -286,6 +286,31 @@ let s:rgb_defaults = { "lightred"     : "#FFBBBB",
                      \ "gray90"       : "#E5E5E5",
                      \ "grey90"       : "#E5E5E5" }
 
+" Colors that vim will use by name in one of the default schemes, either for
+" bg=light or for bg=dark.  This lets us avoid loading the entire rgb.txt
+" database when the scheme itself doesn't ask for colors by name.
+let s:rgb_presets = { "black"         : "#000000",
+                     \ "blue"         : "#0000ff",
+                     \ "brown"        : "#a52a2a",
+                     \ "cyan"         : "#00ffff",
+                     \ "darkblue"     : "#00008b",
+                     \ "darkcyan"     : "#008b8b",
+                     \ "darkgrey"     : "#a9a9a9",
+                     \ "darkmagenta"  : "#8b008b",
+                     \ "green"        : "#00ff00",
+                     \ "grey"         : "#bebebe",
+                     \ "grey40"       : "#666666",
+                     \ "grey90"       : "#e5e5e5",
+                     \ "lightblue"    : "#add8e6",
+                     \ "lightcyan"    : "#e0ffff",
+                     \ "lightgrey"    : "#d3d3d3",
+                     \ "lightmagenta" : "#ffbbff",
+                     \ "magenta"      : "#ff00ff",
+                     \ "red"          : "#ff0000",
+                     \ "seagreen"     : "#2e8b57",
+                     \ "white"        : "#ffffff",
+                     \ "yellow"       : "#ffff00" }
+
 " {>2} Find available color names
 " Find the valid named colors.  If it is available, use the "showrgb" program,
 " otherwise use our own copy of rgb.txt (needed on OS X and systems without
@@ -488,16 +513,23 @@ function! s:SetCtermFromGui(hl)
     " Try translating anything but 'fg', 'bg', #rrggbb, and rrggbb from an
     " rgb.txt color to a #rrggbb color
     if val !~? '^[fb]g$' && val !~ '^#\=\x\{6}$'
-      if empty(s:rgb)
-        call s:UpdateRgbHash()
-      endif
       try
-        let val = s:rgb[tolower(val)]
+        " First see if it is in our preset-by-vim rgb list
+        let val = s:rgb_presets[tolower(val)]
       catch
-        if &verbose
-          echomsg "CSApprox: Colorscheme uses unknown color \"" . val . "\""
+        " Then try loading and checking our real rgb list
+        if empty(s:rgb)
+          call s:UpdateRgbHash()
         endif
-        continue
+        try
+          let val = s:rgb[tolower(val)]
+        catch
+          " And then barf if we still haven't found it
+          if &verbose
+            echomsg "CSApprox: Colorscheme uses unknown color \"" . val . "\""
+          endif
+          continue
+        endtry
       endtry
     endif