gpu: exploit pathCache in collectOps

Previously the cache was only filled during gpu-buffer creation,
resulting in extra work on the CPU to transform vertices if the same
shape was used multiple times in the same frame. Cases such as font
rendering was cached already before this change as it is drawn in it's
own op.Ops that is never reset - and thus re-used from one frame
to the next.

Since we are now calling put() twice per frame an update should no
longer panic.

Signed-off-by: Viktor <viktor.ogeman@gmail.com>

gpu/caches.go

@@ -85,9 +85,6 @@ func (r *opCache) get(key ops.Key) (opCacheValue, bool) {
 }
 
 func (r *opCache) put(key ops.Key, val opCacheValue) {
-	if _, exists := r.newRes[key]; exists {
-		panic(fmt.Errorf("key exists, %#v", key))
-	}
 	r.res[key] = val
 	r.newRes[key] = val
 }

gpu/gpu.go

@@ -319,7 +319,7 @@ func (g *GPU) Collect(viewport image.Point, frameOps *op.Ops) {
 		g.cleanupTimer = g.timers.newTimer()
 	}
 	for _, p := range g.drawOps.pathOps {
-		if _, exists := g.drawOps.pathCache.get(p.pathKey); !exists {
+		if v, exists := g.drawOps.pathCache.get(p.pathKey); !exists || v.data == nil {
 			data := buildPath(g.ctx, p.pathVerts)
 			g.drawOps.pathCache.put(p.pathKey, opCacheValue{
 				data:   data,
@@ -746,10 +746,13 @@ loop:
 				auxKey = auxKey.SetTransform(trans)
 				if v, ok := d.pathCache.get(auxKey); ok {
 					// Since the GPU data exists in the cache aux will not be used.
+					// Why is this not used for the offset shapes?
 					op.bounds = v.bounds
 				} else {
 					aux, op.bounds = d.buildVerts(aux, trans)
-					// this will be added to the cache when building the paths later
+					// add it to the cache, without GPU data, so the transform can be
+					// reused.
+					d.pathCache.put(auxKey, opCacheValue{bounds: op.bounds})
 				}
 			} else {
 				aux, op.bounds, _ = d.boundsForTransformedRect(bounds, trans)