message: fix readPixels vertical offset in vk backend
  vulkan expected a vertical offset from the top, but the api is in
  gl convention with the offset from the bottom.
diff: @@ -1487,7 +1487,7 @@ void VulkanDriver::readPixels(Handle<HwRenderTarget> src, uint32_t x, uint32_t y
  },
  .srcOffset = {
  .x = (int32_t) x,
  - .y = (int32_t) y,
  + .y = (int32_t) (srcTarget->getExtent(mContext.currentSurface).height - (height + y)),
  },
  .dstSubresource = {
  .aspectMask = VK_IMAGE_ASPECT_COLOR_BIT,
  