V210 issue fix + correct packing

internal/generator/text.go

@@ -189,15 +189,16 @@ func (o *TextOverlay) pack() {
 // ApplyV210 stamps the pre-packed text tile over a packed v210 frame.
 // The tile occupies whole 16-byte blocks, so this is a row-wise copy.
 func (o *TextOverlay) ApplyV210(dest []byte) error {
-	lastPixel := (o.y+o.h-1)*o.frameW + o.x + o.w - 1
-	if need := (lastPixel/6 + 1) * 16; len(dest) < need {
+	frameStride := V210LineSize(o.frameW)
+	tileStride := (o.w / 6) * 16
+	need := (o.y+o.h-1)*frameStride + o.x/6*16 + tileStride
+	if len(dest) < need {
 		return fmt.Errorf("text: dest %d bytes too small, need %d", len(dest), need)
 	}
-	tileStride := (o.w / 6) * 16
 	for row := 0; row < o.h; row++ {
-		frameBlock := ((o.y+row)*o.frameW + o.x) / 6
+		frameOffset := (o.y+row)*frameStride + o.x/6*16
 		src := o.blocks[row*tileStride : (row+1)*tileStride]
-		copy(dest[frameBlock*16:frameBlock*16+tileStride], src)
+		copy(dest[frameOffset:frameOffset+tileStride], src)
 	}
 	return nil
 }