gpu: build gpu data also when outside window

This commit fixes a bug where a shape first drawn off-screen
and later moved into screen would not display properly. Since we
cache CPU operations (vertex transform / construction) we need to
upload the constructed data to the GPU after it was build, or a later
frame will use non-initialized memory for it's draw call.

Note that this fix removes the optimization of not processing clip
paths outside the screen - but this is assumed to be uncommon except
when it is first drawn off screen to later be moved in (e.g. in a scrolling list)
in which case we do want to upload the data and prepare for that later
call.

This commit also does a few minor clean ups and adds a test case.

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

gpu/caches.go

@@ -28,8 +28,9 @@ type opCache struct {
 type opCacheValue struct {
 	data   pathData
 	bounds f32.Rectangle
-	key    ops.Key
-	keep   bool
+	// the fields below are handled by opCache
+	key  ops.Key
+	keep bool
 }
 
 func newResourceCache() *resourceCache {

gpu/gpu.go

@@ -706,9 +706,9 @@ func (d *drawOps) addClipPath(state *drawState, aux []byte, auxKey ops.Key, boun
 // split a transform into two parts, one which is pur offset and the
 // other representing the scaling, shearing and rotation part
 func splitTransform(t f32.Affine2D) (srs f32.Affine2D, offset f32.Point) {
-	sx, hx, ox, sy, hy, oy := t.Elems()
+	sx, hx, ox, hy, sy, oy := t.Elems()
 	offset = f32.Point{X: ox, Y: oy}
-	srs = f32.NewAffine2D(sx, hx, 0, sy, hy, 0)
+	srs = f32.NewAffine2D(sx, hx, 0, hy, sy, 0)
 	return
 }
 
@@ -752,9 +752,6 @@ loop:
 				auxKey.SetTransform(trans)
 			}
 			state.clip = state.clip.Intersect(op.bounds.Add(off))
-			if state.clip.Empty() {
-				continue
-			}
 			d.addClipPath(&state, aux, auxKey, op.bounds, off)
 			aux = nil
 			auxKey = ops.Key{}