WIP: switch build to SDL 3 (does not compile yet)

This is the first commit of the SDL 1.2 -> SDL 3 migration. It lands
the build-system change and the include-path rewrites; the code does
NOT compile against SDL 3 yet, by design. Stays on this branch until
the migration is finished.

- CMakeLists.txt: find_package(SDL3 CONFIG) + SDL3::SDL3 /
  SDL3_image::SDL3_image. Drops the legacy FindSDL machinery.
- glSDL.h, sprite.c, glSDL.c, filters.c, gfxengine.cpp, audio.c:
  rewrite #include "SDL*.h" -> <SDL3/...> / <SDL3_image/...>.

Known wall: graphics/glSDL.{c,h} collides with SDL 3's surface API
renames (SDL_FreeSurface, SDL_FillRect, SDL_SetColorKey,
SDL_SetClipRect, SDL_bool). The next phase deletes the entire glSDL
shim and rewrites graphics/ against SDL_Renderer / SDL_Texture.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CMakeLists.txt

@@ -35,11 +35,9 @@ check_symbol_exists(_vsnprintf    stdio.h HAVE__VSNPRINTF)
 # ---------------------------------------------------------------------------
 # Third-party libraries
 # ---------------------------------------------------------------------------
-# This codebase targets SDL 1.2 and SDL_image 1.2. CMake's bundled FindSDL
-# / FindSDL_image modules locate these directly; on macOS with Homebrew,
-# `brew install sdl12-compat sdl_image` works.
-find_package(SDL REQUIRED)
-find_package(SDL_image REQUIRED)
+# Migrating to SDL 3. Homebrew: `brew install sdl3 sdl3_image`.
+find_package(SDL3 CONFIG REQUIRED)
+find_package(SDL3_image CONFIG REQUIRED)
 
 if(KOBO_ENABLE_OPENGL)
     find_package(OpenGL)
@@ -122,9 +120,8 @@ target_compile_definitions(kobo_deps INTERFACE
 target_include_directories(kobo_deps INTERFACE
     ${CMAKE_CURRENT_BINARY_DIR}    # for generated aconfig.h
     ${CMAKE_CURRENT_SOURCE_DIR}    # for config.h
-    ${SDL_INCLUDE_DIR}
 )
-target_link_libraries(kobo_deps INTERFACE ${SDL_LIBRARY} ${SDL_IMAGE_LIBRARIES})
+target_link_libraries(kobo_deps INTERFACE SDL3::SDL3 SDL3_image::SDL3_image)
 
 if(HAVE_OPENGL)
     target_compile_definitions(kobo_deps INTERFACE HAVE_OPENGL)

graphics/filters.c

@@ -23,7 +23,7 @@
 #include <math.h>
 #include "logger.h"
 #include "glSDL.h"
-#include "SDL_image.h"
+#include <SDL3_image/SDL_image.h>
 #include "sprite.h"
 #include "filters.h"
 

graphics/gfxengine.cpp

@@ -28,7 +28,7 @@
 
 #include "gfxengine.h"
 #include "filters.h"
-#include "SDL_image.h"
+#include <SDL3_image/SDL_image.h>
 #include "glSDL.h"
 #include "sofont.h"
 #include "window.h"

graphics/glSDL.c

@@ -30,7 +30,7 @@
 #include <math.h>
 
 #if HAS_SDL_OPENGL_H
-#include "SDL_opengl.h"
+#include <SDL3/SDL_opengl.h>
 #else
 #ifdef WIN32
 #include <windows.h>

graphics/glSDL.h

@@ -20,7 +20,7 @@
  */
 
 /* We're still using SDL datatypes here - we just add some stuff. */
-#include "SDL.h"
+#include <SDL3/SDL.h>
 
 /*
  * Ignore the flag from SDL w/ glSDL backend, since we're going

graphics/sprite.c

@@ -30,7 +30,7 @@ TODO: tables as needed when loading banks.
 #include <string.h>
 #include "logger.h"
 #include "glSDL.h"
-#include "SDL_image.h"
+#include <SDL3_image/SDL_image.h>
 #include "sprite.h"
 #include "filters.h"
 

sound/audio.c

@@ -45,8 +45,8 @@
 #include "audiodev.h"
 #endif
 
-#include "SDL.h"
-#include "SDL_audio.h"
+#include <SDL3/SDL.h>
+#include <SDL3/SDL_audio.h>
 #include "logger.h"
 
 #include "a_struct.h"