Implement solid color cmd

Avoids empty fill segment list, which was a minor bug.

Also increase tolerance to 0.25 to juice performance.

piet-gpu/shader/kernel3.comp

@@ -110,7 +110,7 @@ void main() {
             FillItemHeader fill_item = FillItemHeader_read(fill_th.items);
             fill_th.items.offset += FillItemHeader_size;
             // TODO: handle segments == 0 but backdrop != specially, it's a solid tile.
-            if (fill_item.segments.offset != 0 || fill_item.backdrop != 0) {
+            if (fill_item.segments.offset != 0) {
                 PietFill fill = PietItem_Fill_read(item_ref);
                 CmdFill cmd = CmdFill(
                     fill_item.segments.offset,
@@ -120,6 +120,12 @@ void main() {
                 alloc_cmd(cmd_ref, cmd_limit);
                 Cmd_Fill_write(cmd_ref, cmd);
                 cmd_ref.offset += Cmd_size;
+            } else if (fill_item.backdrop != 0) {
+                // TODO: truncate existing cmd list if alpha is opaque
+                PietFill fill = PietItem_Fill_read(item_ref);
+                alloc_cmd(cmd_ref, cmd_limit);
+                Cmd_Solid_write(cmd_ref, CmdSolid(fill.rgba_color));
+                cmd_ref.offset += Cmd_size;
             }
             break;
         }

piet-gpu/shader/kernel4.comp

@@ -106,6 +106,11 @@ void main() {
             alpha = min(abs(area), 1.0);
             rgb = mix(rgb, fg_rgba.rgb, alpha * fg_rgba.a);
             break;
+        case Cmd_Solid:
+            CmdSolid solid = Cmd_Solid_read(cmd_ref);
+            fg_rgba = unpackUnorm4x8(solid.rgba_color).wzyx;
+            rgb = mix(rgb, fg_rgba.rgb, fg_rgba.a);
+            break;
         case Cmd_Jump:
             cmd_ref = CmdRef(Cmd_Jump_read(cmd_ref).new_ref);
             continue;

piet-gpu/src/render_ctx.rs

@@ -39,7 +39,7 @@ pub enum PietGpuBrush {
     Gradient,
 }
 
-const TOLERANCE: f64 = 0.1;
+const TOLERANCE: f64 = 0.25;
 
 impl PietGpuRenderContext {
     pub fn new() -> PietGpuRenderContext {