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 {