configure: do not prefer SDL2 over SDL3

Unpin SDL version 2 - SDL 3 can be now considered idempotent.

Issues mentioned in sdl3:
- sdl_mixer -> replaced with fluidsynth
- Vulkan issues - Vulkan is currently (SDL 3.20) not implicit in either
of platforms; if so, trigger a warning. Also it is unavailable in SDL2.
- p010 corruption in Direct3D 11 or 12 - there is already a workaround
to disable

Author: MartinPulec

configure.ac

@@ -1321,12 +1321,7 @@ if test "$sdl_req" != no; then
                 sdl2_is_compat=yes
         fi
         if test "$sdl_req" = yes || test "$sdl_req" = auto; then
-                # temporarily prefer 2 over 3
-                sdl_req_ver=231
-                # but only if sdl2 is not sdl2_compat
-                if test "$sdl2_is_compat"; then
-                        sdl_req_ver=321
-                fi
+                sdl_req_ver=321
         fi
 
         while test -n "$sdl_req_ver"; do
@@ -1342,9 +1337,6 @@ if test "$sdl_req" != no; then
                 sdl_req_ver=$(printf "$sdl_req_ver" | cut -c2-)
         done
 
-        if test "$sdl_version" = 3 && test ! "$sdl2_is_compat"; then
-                UG_MSG_WARN([Using SDL 3, which is experimental. SDL 2 can be enforced with --enable-sdl=2])
-        fi
         if test "$sdl_version" = 2 && test "$sdl2_is_compat" ; then
                 UG_MSG_WARN([Using SDL3 sdl2_compat, which is not recommended (use 3 directly)])
         fi

src/video_display/sdl3.c

@@ -45,7 +45,7 @@
  * @todo errata (SDL3 vs SDL2)
  * 1. [macOS] Vulkan renderer doesn't work (no matter if linked with MoltenVK or
  * loader)
- * 2. [all platforms] with `renderer=vulkan` - none YCbCr texture works
+ * 2. [all platforms] with `renderer=vulkan` - none of YCbCr textures work
  * (segfaults - wrong pitch/texture?)
  * 3. p010 works just on macOS/Metal, crashes on Vulkan (see previous point)
  * 4. p010 corrupted on d3d[12] - pixfmts skipped in query*() as a workaround