Vulkan: Add initial VkPixelBuffer.

VKPixelBuffer is used by external render engines to allocate buffers
using the same GPU backend that Blender runs in.

In a later stage we should test what exact binding flags are needed. I
expect that it should be able to use as a transfer buffer to copy the
pixels over to a texture using transfer commands.

Pull Request: blender/blender#105741

source/blender/gpu/vulkan/vk_pixel_buffer.hh

@@ -9,9 +9,13 @@
 
 #include "gpu_texture_private.hh"
 
+#include "vk_buffer.hh"
+
 namespace blender::gpu {
 
 class VKPixelBuffer : public PixelBuffer {
+  VKBuffer buffer_;
+
  public:
   VKPixelBuffer(int64_t size);
   void *map() override;