Fix Validation error in blit path that fails for Mali devices

[leegao]

VVL:

Validation Error: [ VUID-VkImageCreateInfo-pNext-02398 ] | MessageID = 0xc94b7919
vkCreateImage(): pCreateInfo->pNext<VkExternalFormatANDROID>.externalFormat (40) is non-zero, but layout is VK_IMAGE_TILING_LINEAR.
The Vulkan spec states: If the pNext chain includes a VkExternalFormatANDROID structure whose externalFormat member is not 0, tiling must be VK_IMAGE_TILING_OPTIMAL (https://docs.vulkan.org/spec/latest/chapters/resources.html#VUID-VkImageCreateInfo-pNext-02398)

Fix:

   if (external_format.externalFormat != 0) {
      image_info.format = VK_FORMAT_UNDEFINED;
      image_info.tiling = VK_IMAGE_TILING_OPTIMAL;
      image_info.usage = VK_IMAGE_USAGE_SAMPLED_BIT;
   }

Fixes #133