gpu: [compute] add compute renderer specific decoding of ops

Until now, the two renderers have shared structures and code for
decoding drawing ops and convert them to GPU-friendly structures.

However, the decoder is tailored to the old renderer and use
structures that poorly fits the new compute renderer.

This change copies the decoder and specializes the copy for the compute
renderer, avoiding a round-trip through the old renderer decoder.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/caches.go

@@ -26,8 +26,6 @@ type opCache struct {
 
 type opCacheValue struct {
 	data pathData
-	// computePath is the encoded path for compute.
-	computePath encoder
 
 	bounds f32.Rectangle
 	// the fields below are handled by opCache