gpu/internal/driver,gpu/headless: don't y-axis flip OpenGL ReadPixels images

The CPU fallback of the compute renderer needs ReadPixels data in OpenGL
format (origin at bottom left). Unfortunately, the OpenGL driver
automatically mirrors images in the Y-axis to match the top left origin
image.RGBA.

Remove the mirroring from the driver and introduce a DownloadImage to
restore the old behaviour.

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

gpu/internal/opengl/backend.go

@@ -154,6 +154,7 @@ func newOpenGLDevice(api driver.OpenGL) (driver.Device, error) {
 		alphaTriple: alphaTripleFor(ver),
 		srgbaTriple: srgbaTriple,
 	}
+	b.feats.BottomLeftOrigin = true
 	if ffboErr == nil {
 		b.feats.Features |= driver.FeatureFloatRenderTargets
 	}
@@ -771,26 +772,9 @@ 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(), gl.RGBA, gl.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 driver.Framebuffer) {
 	b.funcs.BindFramebuffer(gl.FRAMEBUFFER, fbo.(*gpuFramebuffer).obj)
 }