Allow a few EXT & IHV vulkan extensions

* VK_EXT_astc_decode_mode
* VK_EXT_swapchain_colorspace - this will cause slight artifacts as we won't
  interpret pixels in the right color space, but it's minor.
* VK_EXT_validation_cache - we don't serialise this but we do allow it during
  capture and pass-through straight. We don't wrap the VkValidationCacheEXT
  object at all which makes support simpler.
* VK_EXT_external_memory_dma_buf - this is just a new memory type for external
  memory, we don't need to do anything special to handle it.
* VK_AMD_mixed_attachment_samples & VK_AMD_shader_core_properties - these are
  no-op once enabled
* Many shader-only extensions:
  - VK_AMD_gpu_shader_int16
  - VK_AMD_shader_fragment_mask
  - VK_AMD_image_load_store_lod
  - VK_AMD_texture_gather_bias_lod
  - VK_NV_compute_shader_derivatives
  - VK_NV_fragment_shader_barycentric
  - VK_NV_geometry_shader_passthrough
  - VK_NV_sample_mask_override_coverage
  - VK_NV_shader_image_footprint
  - VK_NV_shader_subgroup_partitioned
  - VK_NV_viewport_array2