make the brush box work under the new GPU renderer

the brush preview will need separate treatment, but it shouldn't be too difficult

static/index.css

@@ -283,6 +283,8 @@ rkgk-reticle-cursor {
 rkgk-brush-box {
     --button-size: 56px;
 
+    position: relative;
+
     height: var(--height);
     padding: 12px;
 
@@ -316,7 +318,7 @@ rkgk-brush-box {
                 border-color: var(--color-brand-blue);
             }
 
-            & > rkgk-brush-preview {
+            & > .preview {
                 width: var(--button-size);
                 aspect-ratio: 1 / 1;
                 background: none;
@@ -352,6 +354,18 @@ rkgk-brush-box {
             display: flex;
         }
     }
+
+    & > canvas {
+        position: absolute;
+        left: 0;
+        top: 0;
+        width: 100%;
+        margin: 12px;
+
+        pointer-events: none;
+
+        image-rendering: pixelated;
+    }
 }
 
 /* Code editor */