Load SDL2 symbols at runtime

Instead of trying to link SDL2 portably at compile time, which requires
the help of CMake and leads to a bunch of ugly #ifdef gunk, we now look
for a shared library at runtime, and load only the symbols we actually
need.
A mostly gutted set of SDL headers is included, purely to provide a few
types for more ergonomic use of the SDL2 API. This bloats the source
tree by about ~160K unfortunately.

CMakeLists.txt

@@ -95,22 +95,6 @@ if (NO_LOGO)
 	target_compile_definitions(c-ray PRIVATE -DNO_LOGO)
 endif()
 
-if (NOT NO_SDL2)
-	find_package(SDL2 QUIET)
-
-	if (SDL2_FOUND)
-		message(STATUS "SDL2 found! Enabling UI components.")
-		target_link_libraries(c-ray PRIVATE ${SDL2_LIBRARY})
-		target_include_directories(c-ray PRIVATE ${SDL2_INCLUDE_DIR})
-		target_compile_definitions(c-ray PRIVATE -DCRAY_SDL_ENABLED)
-	else()
-		message(STATUS "SDL2 not found! You should install it, it'll be awesome! See README for instructions.")
-	endif()
-else()
-	message(STATUS "SDL2 explicitly disabled, disabling UI components")
-	unset(NO_SDL2 CACHE)
-endif()
-
 if (TESTING)
 	message(STATUS "Enabling test suite.")
 	target_compile_definitions(c-ray PRIVATE -DCRAY_TESTING)

src/renderer/renderer.c

@@ -147,7 +147,7 @@ struct texture *renderFrame(struct renderer *r) {
 
 		//Gather and maintain this average constantly.
 		if (!r->state.threadStates[0].paused) {
-			win_update(r->sdl, r, output);
+			if (r->sdl) win_update(r->sdl, r, output);
 			for (int t = 0; t < localThreadCount; ++t) {
 				avgSampleTime += r->state.threadStates[t].avgSampleTime;
 			}

src/renderer/renderer.h

@@ -100,7 +100,7 @@ struct renderer {
 	struct state state;  //Internal state
 	struct prefs prefs;  //User prefs
 	char *sceneCache;    //Packed scene data that can be passed to workers
-	struct window *sdl;  //FIXME: Temporarily here, I guess.
+	struct sdl_window *sdl; //FIXME: Temporarily here
 };
 
 //Initialize a new renderer