VK-to-GL: Resource sharing

This change allows sharing of image resources between different vrend/vkr contexts. Consider an instance of QEMU which uses OpenGL for rendering its window: running a Vulkan application inside the Guest would generate a Vulkan resource on the host, which should be drawn by QEMU OpenGL frontend, which means we should import that Vulkan resource to a GL one for the current context.

TODO:

• Find the right format for the imported image.
• Stabilize BLOB resources API.