vulkan: Initial Vulkan support!

This work was done by Jacob Lifshay and Mark Callow; I'm just merging it
into revision control.

--HG--
extra : rebase_source : cb3bb332146125366b5242c0d7444f44638733d9
extra : amend_source : c72ebb6d3a92efc3fdd085ce3b6d4b1d7f573cc9

cmake/sdlchecks.cmake

@@ -1153,6 +1153,25 @@ macro(CheckRPI)
   endif(VIDEO_RPI)
 endmacro(CheckRPI)
 
+macro(CheckVulkanHeaders)
+  if(VIDEO_VULKAN)
+    # ${VULKAN_SDK} could be unset during the first configure run with
+    # cmake-gui resulting in vulkan.h not being found. If it's been
+    # subsequently changed, unset is necessary to ensure check is run again.
+    unset(HAVE_VULKAN_H CACHE)
+    # Prefer ${VULKAN_SDK} header
+    set(CMAKE_REQUIRED_INCLUDES "${VULKAN_SDK}/include")
+    check_include_file("vulkan/vulkan.h" HAVE_VULKAN_H)
+    if(HAVE_VULKAN_H)
+      list(APPEND EXTRA_CFLAGS "-I${VULKAN_SDK}/include")
+    else()
+      # Check system includes.
+      unset(HAVE_VULKAN_H CACHE)
+      check_include_file("vulkan/vulkan.h" HAVE_VULKAN_H)
+    endif()
+  endif()
+endmacro(CheckVulkanHeaders)
+
 # Requires:
 # - EGL
 # - PkgCheckModules