app/headless,gpu/gl: make ReadPixels y-flipping backend specific

The Direct3D backend doesn't need y-flipping, so don't do it unconditionally in
package app/headless.

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

gpu/gl/backend.go

@@ -640,9 +640,26 @@ func (f *gpuFramebuffer) ReadPixels(src image.Rectangle, pixels []byte) error {
 		return errors.New("unexpected RGBA size")
 	}
 	f.backend.funcs.ReadPixels(src.Min.X, src.Min.Y, src.Dx(), src.Dy(), RGBA, UNSIGNED_BYTE, pixels)
+	// OpenGL origin is in the lower-left corner. Flip the image to
+	// match.
+	flipImageY(src.Dx()*4, src.Dy(), pixels)
 	return glErr(f.backend.funcs)
 }
 
+func flipImageY(stride int, height int, pixels []byte) {
+	// Flip image in y-direction. OpenGL's origin is in the lower
+	// left corner.
+	row := make([]uint8, stride)
+	for y := 0; y < height/2; y++ {
+		y1 := height - y - 1
+		dest := y1 * stride
+		src := y * stride
+		copy(row, pixels[dest:])
+		copy(pixels[dest:], pixels[src:src+len(row)])
+		copy(pixels[src:], row)
+	}
+}
+
 func (b *Backend) BindFramebuffer(fbo backend.Framebuffer) {
 	b.funcs.BindFramebuffer(FRAMEBUFFER, fbo.(*gpuFramebuffer).obj)
 }