driver: Refactoring: Remove Graphics.Flush

Updates #226

internal/driver/graphics.go

@@ -28,7 +28,6 @@ type Graphics interface {
 	SetWindow(window unsafe.Pointer)
 	SetTransparent(transparent bool)
 	SetVertices(vertices []float32, indices []uint16)
-	Flush()
 	NewImage(width, height int) (Image, error)
 	NewScreenFramebufferImage(width, height int) (Image, error)
 	Reset() error

internal/graphicscommand/command.go

@@ -274,10 +274,6 @@ func (q *commandQueue) Flush() {
 			// introduced than drawTrianglesCommand.
 			indexOffset += c.NumIndices()
 		}
-		if 0 < nc {
-			// Call glFlush to prevent black flicking (especially on Android (#226) and iOS).
-			theGraphicsDriver.Flush()
-		}
 		cs = cs[nc:]
 	}
 	theGraphicsDriver.End()

internal/graphicsdriver/metal/driver.go

@@ -364,10 +364,6 @@ func (d *Driver) SetVertices(vertices []float32, indices []uint16) {
 	})
 }
 
-func (d *Driver) Flush() {
-	// On Metal, flushing command buffers only once is enough except for manipulating pixels. Do not call flush.
-}
-
 func (d *Driver) flush(wait bool, present bool) {
 	d.t.Call(func() error {
 		if d.cb == (mtl.CommandBuffer{}) {

internal/graphicsdriver/opengl/driver.go

@@ -47,7 +47,9 @@ func (d *Driver) Begin() {
 }
 
 func (d *Driver) End() {
-	// Do nothing.
+	// Call glFlush to prevent black flicking (especially on Android (#226) and iOS).
+	// TODO: examples/sprites worked without this. Is this really needed?
+	d.context.flush()
 }
 
 func (d *Driver) SetWindow(window unsafe.Pointer) {
@@ -128,10 +130,6 @@ func (d *Driver) Draw(indexLen int, indexOffset int, mode driver.CompositeMode,
 	return nil
 }
 
-func (d *Driver) Flush() {
-	d.context.flush()
-}
-
 func (d *Driver) SetVsyncEnabled(enabled bool) {
 	// Do nothing
 }