[WIP] VL backend for the new SDL3 GPU API

This is a very early proof-of-concept for SDL3 GPU support, for the API
mentioned in libsdl-org/SDL#9312

It's currently lacking:
- A decent cleanup and error handling.
- Proper separation from the vulkan backend (I broke the vulkan shaders
  to implement this)
- A Makefile implementation (CMake only for now)
- Some strange framebuffer bugs on hasvk/wayland/HSW -- probably a
  driver bug
- Not once tested with anything other than the Vulkan backend, which has
  a habit of allowing a lot of things other backends don't.

CMakeLists.txt

@@ -176,6 +176,37 @@ elseif(RENDERER STREQUAL "sdl3")
 		${SDL3_INCLUDE_DIRS}
 	)
 	add_definitions(-DWITH_SDL=3)
+elseif(RENDERER STREQUAL "sdl3gpu")
+	message(STATUS "Using SDL3 gpu backend")
+	find_package(SDL3 REQUIRED)
+	add_custom_command(OUTPUT id_vl_vk_vert.h
+		COMMAND glslc -mfmt=c
+			${CMAKE_CURRENT_SOURCE_DIR}/src/id_vl_vk_shader.vert
+			-o id_vl_vk_vert.h
+		DEPENDS src/id_vl_vk_shader.vert
+	)
+
+	add_custom_command(OUTPUT id_vl_vk_frag.h
+		COMMAND glslc -mfmt=c
+			${CMAKE_CURRENT_SOURCE_DIR}/src/id_vl_vk_shader.frag
+			-o id_vl_vk_frag.h
+		DEPENDS src/id_vl_vk_shader.frag
+	)
+	set(OMNISPEAK_PLATFORM_SRCS
+		src/id_in_sdl3.c
+		src/id_sd_sdl3.c
+		src/id_vl_sdl3gpu.c
+		${CMAKE_CURRENT_BINARY_DIR}/id_vl_vk_vert.h
+		${CMAKE_CURRENT_BINARY_DIR}/id_vl_vk_frag.h
+	)
+	set(OMNISPEAK_PLATFORM_LIBRARIES
+		${SDL3_LIBRARIES}
+	)
+	include_directories(
+		${SDL3_INCLUDE_DIRS}
+		${CMAKE_CURRENT_BINARY_DIR}
+	)
+	add_definitions(-DWITH_SDL=3)
 else()
 	message(WARNING "Using NULL platform layer.")
 	set(OMNISPEAK_PLATFORM_SRCS

src/id_in_sdl3.c

@@ -25,6 +25,10 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 #include <SDL3/SDL.h>
 #include <string.h>
 
+extern SDL_Window *vl_sdl3gpu_window;
+#define vl_sdl3_window vl_sdl3gpu_window
+
+
 #define IN_MAX_JOYSTICKS 2
 
 SDL_Joystick *in_joysticks[IN_MAX_JOYSTICKS];