font/freetype: Rewrite monoToGrayscale algorithm

The original version had issues converting properly and caused broken
glyphs. This version tries to be as simple as possible in order to make
it easy to understand. I haven't measured the performance but in
practice this will only happen during the first render of the glyph
after a face change (i.e. during launch or when changing font size).

src/font/face/freetype_convert.zig

@@ -43,26 +43,14 @@ pub fn monoToGrayscale(alloc: Allocator, bm: Bitmap) Allocator.Error!Bitmap {
     var buf = try alloc.alloc(u8, bm.width * bm.rows);
     errdefer alloc.free(buf);
 
-    // width divided by 8 because each byte has 8 pixels. This is therefore
+    for (0..bm.rows) |y| {
-    // the number of bytes in each row.
+        const row_offset = y * @as(usize, @intCast(bm.pitch));
-    const bytes_per_row = bm.width >> 3;
+        for (0..bm.width) |x| {
-
+            const byte_offset = row_offset + @divTrunc(x, 8);
-    var source_i: usize = 0;
+            const mask = @as(u8, 1) << @intCast(7 - (x % 8));
-    var target_i: usize = 0;
+            const bit: u8 = @intFromBool((bm.buffer[byte_offset] & mask) != 0);
-    var i: usize = bm.rows;
+            buf[y * bm.width + x] = bit * 255;
-    while (i > 0) : (i -= 1) {
-        var j: usize = bytes_per_row;
-        while (j > 0) : (j -= 1) {
-            var bit: u4 = 8;
-            while (bit > 0) : (bit -= 1) {
-                const mask = @as(u8, 1) << @as(u3, @intCast(bit - 1));
-                const bitval: u8 = if (bm.buffer[source_i + (j - 1)] & mask > 0) 0xFF else 0;
-                buf[target_i] = bitval;
-                target_i += 1;
-            }
         }
-
-        source_i += @intCast(bm.pitch);
     }
 
     var copy = bm;