[AD] grab_font_from_bitmap speedup patch

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

See http://www.allegro.cc/forums/thread/589885 for background information.
Essentially, the current implementation scans the entire bitmap for the 
presence of an alpha channel for each character in the font - which can 
slow things down pretty badly.
The attached patch (untested) only does the scan once for each call to 
grab_font_from_bitmap.

Evert

Index: src/fontbmp.c
===================================================================
--- src/fontbmp.c	(revision 7709)
+++ src/fontbmp.c	(working copy)
@@ -24,29 +24,18 @@
 /* state information for the bitmap font importer */
 static int import_x = 0;
 static int import_y = 0;
+static int font_outline_colour = 0;
 
 
 
 /* splits bitmaps into sub-sprites, using regions bounded by col #255 */
 static void font_find_character(BITMAP *bmp, int *x, int *y, int *w, int *h)
 {
-   int c;
-
-   if (_bitmap_has_alpha(bmp)) {
-      c = getpixel(bmp, 0, 0);
-   }
-   else if (bitmap_color_depth(bmp) == 8) {
-      c = 255;
-   }
-   else {
-      c = makecol_depth(bitmap_color_depth(bmp), 255, 255, 0);
-   }
-
    /* look for top left corner of character */
-   while ((getpixel(bmp, *x, *y) != c) || 
-	  (getpixel(bmp, *x+1, *y) != c) ||
-	  (getpixel(bmp, *x, *y+1) != c) ||
-	  (getpixel(bmp, *x+1, *y+1) == c)) {
+   while ((getpixel(bmp, *x, *y) != font_outline_colour) || 
+	  (getpixel(bmp, *x+1, *y) != font_outline_colour) ||
+	  (getpixel(bmp, *x, *y+1) != font_outline_colour) ||
+	  (getpixel(bmp, *x+1, *y+1) == font_outline_colour)) {
       (*x)++;
       if (*x >= bmp->w) {
 	 *x = 0;
@@ -61,15 +50,15 @@
 
    /* look for right edge of character */
    *w = 0;
-   while ((getpixel(bmp, *x+*w+1, *y) == c) &&
-	  (getpixel(bmp, *x+*w+1, *y+1) != c) &&
+   while ((getpixel(bmp, *x+*w+1, *y) == font_outline_colour) &&
+	  (getpixel(bmp, *x+*w+1, *y+1) != font_outline_colour) &&
 	  (*x+*w+1 <= bmp->w))
       (*w)++;
 
    /* look for bottom edge of character */
    *h = 0;
-   while ((getpixel(bmp, *x, *y+*h+1) == c) &&
-	  (getpixel(bmp, *x+1, *y+*h+1) != c) &&
+   while ((getpixel(bmp, *x, *y+*h+1) == font_outline_colour) &&
+	  (getpixel(bmp, *x+1, *y+*h+1) != font_outline_colour) &&
 	  (*y+*h+1 <= bmp->h))
       (*h)++;
 }
@@ -225,6 +214,17 @@
    import_x = 0;
    import_y = 0;
 
+   /* Find outline colour for this bitmap */
+   if (_bitmap_has_alpha(bmp)) {
+      font_outline_colour = getpixel(bmp, 0, 0);
+   }
+   else if (bitmap_color_depth(bmp) == 8) {
+      font_outline_colour = 255;
+   }
+   else {
+      font_outline_colour = makecol_depth(bitmap_color_depth(bmp), 255, 255, 0);
+   }
+
    f = _AL_MALLOC(sizeof *f);
    if (end == -1) end = bitmap_font_count(bmp) + begin;