thanks to Exophase for help with coming up with this fix
+ vorr.u16 pixels, pixels, msb_mask
vorr.u16 draw_mask, draw_mask, write_mask
vbif.u16 fb_pixels, pixels, draw_mask
vst1.u16 { fb_pixels }, [ fb_ptr ]
vorr.u16 draw_mask, draw_mask, write_mask
vbif.u16 fb_pixels, pixels, draw_mask
vst1.u16 { fb_pixels }, [ fb_ptr ]
+ vorr.u16 pixels, pixels, msb_mask
vorr.u16 draw_mask, draw_mask, write_mask
vbif.u16 fb_pixels, pixels, draw_mask
vst1.u16 { fb_pixels }, [ fb_ptr ]
vorr.u16 draw_mask, draw_mask, write_mask
vbif.u16 fb_pixels, pixels, draw_mask
vst1.u16 { fb_pixels }, [ fb_ptr ]