demo/backend: add vulkan SDL backend and demo

Based on other SDL demos and GLFW Vulkan demo.

demo/sdl_vulkan/.gitignore

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+src/nuklearshaders/*.spv
+src/nuklear_sdl_vulkan.h
+shaders/*.spv

demo/sdl_vulkan/Makefile

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+# Install
+BIN = demo
+
+# Flags
+CFLAGS += -std=c89 -Wall -Wextra -pedantic -fsanitize=address -O2
+
+SRC = main.c
+OBJ = $(SRC:.c=.o)
+
+ifeq ($(OS),Windows_NT)
+BIN := $(BIN).exe
+LIBS = -lsdl2 -lvulkan -lm
+else
+	UNAME_S := $(shell uname -s)
+	SDL2 := $(shell pkg-config --libs sdl2)
+	LIBS = $(SDL2) -lvulkan -lm
+endif
+
+
+$(BIN): shaders/demo.vert.spv shaders/demo.frag.spv
+	@mkdir -p bin
+	rm -f bin/$(BIN) $(OBJS)
+	$(CC) $(SRC) $(CFLAGS) -o bin/$(BIN) $(LIBS)
+
+shaders/demo.vert.spv: shaders/demo.vert
+	glslc --target-env=vulkan shaders/demo.vert -o shaders/demo.vert.spv
+
+shaders/demo.frag.spv: shaders/demo.frag
+	glslc --target-env=vulkan shaders/demo.frag -o shaders/demo.frag.spv

demo/sdl_vulkan/README.md

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+# nuklear sdl vulkan
+
+## Theory of operation
+
+The nuklear SDL vulkan integration creates an independent graphics pipeline that will render the nuklear UI to separate render targets.
+The application is responsible to fully manage these render targets. So it must ensure they are properly sized (and resized when requested).
+
+Furthermore it is assumed that you will have a swap chain in place and the number of nuklear overlay images and number of swap chain images match.
+
+This is how you can integrate it in your application:
+
+```
+/*
+Setup: overlay_images have been created and their number match with the number
+of the swap_chain_images of your application. The overlay_images in this
+example have the same format as your swap_chain images (optional)
+*/
+    struct nk_context *ctx = nk_sdl_init(
+        demo.win, demo.device, demo.physical_device, demo.indices.graphics,
+        demo.overlay_image_views, demo.swap_chain_images_len,
+        demo.swap_chain_image_format, 0,
+        MAX_VERTEX_BUFFER, MAX_ELEMENT_BUFFER);
+[...]
+/*
+in your draw loop draw you can then render to the overlay image at
+`image_index`
+your own application can then wait for the semaphore and produce
+the swap_chain_image at `image_index`
+this should simply sample from the overlay_image (see example)
+*/
+nk_semaphore semaphore =
+    nk_sdl_render(demo.graphics_queue, image_index,
+                    demo.image_available, NK_ANTI_ALIASING_ON);
+    if (!render(&demo, &bg, nk_semaphore, image_index)) {
+        fprintf(stderr, "render failed\n");
+        return false;
+    }
+```
+
+You must call `nk_sdl_resize` whenever the size of the overlay_images resize.
+
+## Using images
+
+Images can be used by providing a VkImageView as an nk_image_ptr to nuklear:
+
+```
+img = nk_image_ptr(demo.demo_texture_image_view);
+```
+
+Note that they must have SHADER_READ_OPTIMAL layout
+
+It is currently not possible to specify how they are being sampled. The nuklear SDL vulkan integration uses a fixed sampler that does linear filtering.