font/sprite: fix horizontal dash drawing math for cells too small

Fixes #143

There were multiple issues with the previous calculation. First, we
expected dash width COULD be negative and protected against it, but our
dash width calculation type was unsigned! With the proper font metrics,
this led to an underflow safey panic.

Second, as part of the dash width calculation, we are tryign to downstep
the size of the gaps so we can try to fit dashes within a cell. But we
were not using those adjusted gap sizes. With the proper font metrics,
this could lead to an assertion failure seen in #143.

This fixes the calculations. They produce the same numbers, but do so in
a more Zig-idiomatic way while fixing the above two issues.

src/font/sprite/Box.zig

@@ -2414,7 +2414,7 @@ fn draw_dash_horizontal(
     canvas: *font.sprite.Canvas,
     count: u8,
     thick_px: u32,
-    gap: u32,
+    desired_gap: u32,
 ) void {
     assert(count >= 2 and count <= 4);
 
@@ -2422,23 +2422,35 @@ fn draw_dash_horizontal(
     // "- - -" => 2 gaps
     const gap_count = count - 1;
 
-    // Determine the width of our dashes
-    const dash_width = dash_width: {
-        var gap_i = gap;
-        var dash_width = (self.width - (gap_count * gap_i)) / count;
-        while (dash_width <= 0 and gap_i > 1) {
-            gap_i -= 1;
-            dash_width = (self.width - (gap_count * gap_i)) / count;
+    // Determine the width of each dash and the gap between them. We try
+    // to have gap match desired_gap but if our cell is too small then we
+    // have to bring it down.
+    const adjusted: struct {
+        dash_width: u32,
+        gap: u32,
+    } = adjusted: {
+        for (0..desired_gap) |i| {
+            const gap_width: u32 = desired_gap - @as(u32, @intCast(i));
+            const total_gap_width: u32 = gap_count * gap_width;
+
+            // This would make a negative and overflow our u32. A negative
+            // dash width is not allowed so we keep trying to fit it.
+            if (total_gap_width >= self.width) continue;
+
+            break :adjusted .{
+                .dash_width = (self.width - total_gap_width) / count,
+                .gap = gap_width,
+            };
         }
 
-        // If we can't fit any dashes then we just render a horizontal line.
-        if (dash_width <= 0) {
-            self.hline_middle(canvas, .light);
-            return;
-        }
-
-        break :dash_width dash_width;
+        // In this case, there is no combination of gap width and dash
+        // width that would fit our desired number of dashes, so we just
+        // draw a horizontal line.
+        self.hline_middle(canvas, .light);
+        return;
     };
+    const dash_width = adjusted.dash_width;
+    const gap = adjusted.gap;
 
     // Our total width should be less than our real width
     assert(count * dash_width + gap_count * gap <= self.width);