gpu: Merge GPU.Collect and GPU.Frame

There's no meaningful reason to have them separate. The intention was to
enable rendering concurrent with other processing, but that's gaining
framerate at the expense of input latency and complicating ImageOp
semantics.

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

op/paint/paint.go

@@ -16,9 +16,6 @@ import (
 )
 
 // ImageOp sets the brush to an image.
-//
-// Note: the ImageOp may keep a reference to the backing image.
-// See NewImageOp for details.
 type ImageOp struct {
 	uniform bool
 	color   color.NRGBA
@@ -47,9 +44,7 @@ type LinearGradientOp struct {
 type PaintOp struct {
 }
 
-// NewImageOp creates an ImageOp backed by src. See
-// gioui.org/io/system.FrameEvent for a description of when data
-// referenced by operations is safe to re-use.
+// NewImageOp creates an ImageOp backed by src.
 //
 // NewImageOp assumes the backing image is immutable, and may cache a
 // copy of its contents in a GPU-friendly way. Create new ImageOps to