Shareable Vulkan image allocation on Linux relies on the dma-buf
interface, which is a Linux-specific thing. Therefore, we should only be
compiling it (and any code that uses it) on Linux. This change adds
preprocessor guards to do that. Enabling similar functionality on other
operating systems will need to leverage analogous interfaces on those
platforms, e.g. win32 handles on Windows.
All Vulkan image code will now be guarded by the USE_VULKAN_IMAGES
preprocessor definition, currently enabled on Linux if Vulkan is
available. Additionally, we shuffle around some code in
OpenGLContext.cpp to simplify the preprocessor conditionals.
This introduces the ability to allocate Vulkan images which can be
shared across graphics APIs or across processes using the Linux dma-buf
interface.
The create_shareable_vulkan_image function takes a VulkanContext, image
width, height, and format, and an array of DRM format modifiers
representing image memory layouts accepted by the caller. The function
will intersect this list with the list of modifiers supported by the
Vulkan implementation, ensuring the resulting image uses one of those
layouts (exactly which one is up to the implementation). The function
will return a VulkanImage object, which is reference-counted and
encapsulates the VkImage itself as well as the backing memory
allocation. It also stores various image parameters such as usage,
tiling, etc.
The VulkanImage::get_dma_buf_fd function will create a file descriptor
representing the image's backing dma-buf which can then be imported by a
different API or sent over a unix domain socket.
