Add new demo: sdl3_renderer

This is a highly updated demo for libSDL3. It fixes a lot of long
occuring issues and brings some improvements specific to SDL3 API.
The origin and development of this demo is quite long.
The exact track of development is something like this:
`Nuklear/demo/sdl_renderer` -> #772 -> #779 + #825 -> _this_ commit...
You may wish to read the linked PRs in order to get the full context.
Commit message is too small to describe everything. I'm sorry...

Author: sleeptightAnsiC

demo/sdl3_renderer/Makefile

@@ -0,0 +1,41 @@
+# this Makefile is specific to GNU Make and GCC'compatible compilers
+
+PKG_CONFIG ?= $(shell command -v pkg-config)
+ifeq (,$(PKG_CONFIG))
+    $(error missing pkg-config utility!)
+endif
+
+PKG_SDL3 ?= sdl3
+ifeq (,$(shell $(PKG_CONFIG) --path $(PKG_SDL3)))
+    $(error $(PKG_CONFIG) could not find: $(PKG_SDL3))
+endif
+
+OS ?= $(shell uname -s)
+BINEXT-Windows_NT = .exe
+BINEXT ?= $(BINEXIT-$(OS))
+
+TEMPDIR ?= ./bin
+BIN ?= $(TEMPDIR)/demo$(BINEXT)
+
+CFLAGS += -std=c89 -Wall -Wextra -Wpedantic
+CFLAGS += -O2
+#CFLAGS += -O0 -g
+#CFLAGS += -fsanitize=address
+#CFLAGS += -fsanitize=undefined
+CFLAGS += $(shell $(PKG_CONFIG) $(PKG_SDL3) --cflags)
+
+LIBS += -lm
+LIBS += $(shell $(PKG_CONFIG) $(PKG_SDL3) --libs)
+
+DEP ?= $(BIN).d
+
+SRC = main.c
+
+$(BIN):
+	mkdir -p $(dir $@)
+	$(CC) $(SRC) -o $@ -MD -MF $(DEP) $(CFLAGS) $(LIBS)
+
+$(BIN): $(SRC) ./Makefile ./nuklear_sdl3_renderer.h ./../../nuklear.h
+
+-include $(DEP)
+