internal/graphicsdriver/metal: call PresentDrawable only when necessary

internal/graphicscommand/command.go

@@ -293,6 +293,7 @@ func (q *commandQueue) flush() error {
 	}
 
 	theGraphicsDriver.Begin()
+	var present bool
 	cs := q.commands
 	for len(cs) > 0 {
 		nv := 0
@@ -308,6 +309,9 @@ func (q *commandQueue) flush() error {
 				}
 				nv += dtc.numVertices()
 				ne += dtc.numIndices()
+				if dtc.dst.screen {
+					present = true
+				}
 			}
 			nc++
 		}
@@ -331,7 +335,7 @@ func (q *commandQueue) flush() error {
 		}
 		cs = cs[nc:]
 	}
-	theGraphicsDriver.End()
+	theGraphicsDriver.End(present)
 
 	// Release the commands explicitly (#1803).
 	// Apparently, the part of a slice between len and cap-1 still holds references.

internal/graphicsdriver/graphics.go

@@ -46,7 +46,7 @@ type Uniform struct {
 
 type Graphics interface {
 	Begin()
-	End()
+	End(present bool)
 	SetTransparent(transparent bool)
 	SetVertices(vertices []float32, indices []uint16)
 	NewImage(width, height int) (Image, error)

internal/graphicsdriver/metal/graphics_darwin.go

@@ -369,8 +369,8 @@ func (g *Graphics) Begin() {
 	g.pool = C.allocAutoreleasePool()
 }
 
-func (g *Graphics) End() {
+func (g *Graphics) End(present bool) {
-	g.flushIfNeeded(true)
+	g.flushIfNeeded(present)
 	g.screenDrawable = ca.MetalDrawable{}
 	C.releaseAutoreleasePool(g.pool)
 	g.pool = nil

internal/graphicsdriver/opengl/graphics.go

@@ -60,7 +60,7 @@ func (g *Graphics) Begin() {
 	// Do nothing.
 }
 
-func (g *Graphics) End() {
+func (g *Graphics) End(present bool) {
 	// Call glFlush to prevent black flicking (especially on Android (#226) and iOS).
 	// TODO: examples/sprites worked without this. Is this really needed?
 	g.context.flush()