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/driver/driver.go

@@ -138,8 +138,11 @@ type DepthFunc uint8
 type Features uint
 
 type Caps struct {
-	Features       Features
-	MaxTextureSize int
+	// BottomLeftOrigin is true if the driver has the origin in the lower left
+	// corner. The OpenGL driver returns true.
+	BottomLeftOrigin bool
+	Features         Features
+	MaxTextureSize   int
 }
 
 type Program interface {
@@ -233,6 +236,33 @@ func (f Features) Has(feats Features) bool {
 	return f&feats == feats
 }
 
+func DownloadImage(d Device, f Framebuffer, r image.Rectangle) (*image.RGBA, error) {
+	img := image.NewRGBA(r)
+	if err := f.ReadPixels(r, img.Pix); err != nil {
+		return nil, err
+	}
+	if d.Caps().BottomLeftOrigin {
+		// OpenGL origin is in the lower-left corner. Flip the image to
+		// match.
+		flipImageY(r.Dx()*4, r.Dy(), img.Pix)
+	}
+	return img, nil
+}
+
+func flipImageY(stride, 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 UploadImage(t Texture, offset image.Point, img *image.RGBA) {
 	var pixels []byte
 	size := img.Bounds().Size()