Enable "debug" optimizations in make develop

Enhances some warnings as well as the sanitizers (Clang especially complained about it)
The `-f*` flags are to get better stack traces out of the sanitizers, as recommended
by Clang's docs: https://clang.llvm.org/docs/AddressSanitizer.html#usage

GCC's docs claim that these optimizations should not hinder the debugging
experience, and Clang's don't mention optimization flags at all.

CMakeLists.txt

@@ -42,16 +42,18 @@ else()
                   -fsanitize=alignment -fsanitize=null -fsanitize=address)
     add_compile_options(${SAN_FLAGS})
     link_libraries(${SAN_FLAGS})
+    # A non-zero optimization level is desired in debug mode, but allow overriding it nonetheless
+    # TODO: this overrides anything previously set... that's a bit sloppy!
+    set(CMAKE_C_FLAGS_DEBUG "-g -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls" CACHE STRING "" FORCE)
   endif()
 
   if(MORE_WARNINGS)
     add_compile_options(-Werror -Wextra
                         -Walloc-zero -Wcast-align -Wcast-qual -Wduplicated-branches -Wduplicated-cond
-                        -Wfloat-equal -Winline -Wlogical-op -Wnested-externs -Wold-style-definition
-                        -Wshift-overflow=2
-                        -Wstrict-overflow=5 -Wstrict-prototypes -Wundef -Wuninitialized -Wunused
+                        -Wfloat-equal -Winline -Wlogical-op -Wnested-externs -Wnull-dereference
+                        -Wold-style-definition -Wshift-overflow=2 -Wstrict-overflow=5
+                        -Wstrict-prototypes -Wstringop-overflow=4 -Wundef -Wuninitialized -Wunused
                         -Wshadow # TODO: -Wshadow=compatible-local ?
-                        -Wnull-dereference -Wstringop-overflow=4 # TODO: would work better with optimizations
                         -Wno-sign-compare # TODO: fix those warnings
                         -Wformat=2 -Wformat-overflow=2 -Wformat-truncation=1
                         -Wno-format-nonliteral # We have a couple of "dynamic" prints

Makefile

@@ -230,7 +230,7 @@ develop:
 		-fsanitize=signed-integer-overflow -fsanitize=bounds \
 		-fsanitize=object-size -fsanitize=bool -fsanitize=enum \
 		-fsanitize=alignment -fsanitize=null -fsanitize=address" \
-		CFLAGS="-ggdb3 -O0"
+		CFLAGS="-ggdb3 -Og -fno-omit-frame-pointer -fno-optimize-sibling-calls"
 
 # Targets for the project maintainer to easily create Windows exes.
 # This is not for Windows users!

src/gfx/makepng.c

@@ -95,6 +95,9 @@ void output_png_file(const struct Options *opts,
 	if (!f)
 		err("Opening output png file '%s' failed", outfile);
 
+	if (opts->debug)
+		free(outfile);
+
 	img.png = png_create_write_struct(PNG_LIBPNG_VER_STRING,
 					  NULL, NULL, NULL);
 	if (!img.png)
@@ -135,9 +138,6 @@ void output_png_file(const struct Options *opts,
 
 	png_destroy_write_struct(&img.png, &img.info);
 	fclose(f);
-
-	if (opts->debug)
-		free(outfile);
 }
 
 void destroy_raw_image(struct RawIndexedImage **raw_image_ptr_ptr)