fix(sdl): build SDL backend on all host platforms

Enable the preview.sdl target for any host build, not just Windows, so
the SDL DirectDisplay can be used from Linux/macOS host applications.

- gate preview/sdl on EMIL_HOST_BUILD instead of EMIL_BUILD_WIN
- avoid taking the address of a temporary SDL_Rect in Convert(), which
  fails to compile under GCC/Clang with -Werror
- drop a duplicate Colour.hpp include

Author: XavierBernhardt-Philips

preview/sdl/CMakeLists.txt

@@ -1,4 +1,4 @@
-if (EMIL_BUILD_WIN AND PREVIEW_INCLUDE_SDL)
+if (EMIL_HOST_BUILD AND PREVIEW_INCLUDE_SDL)
     include(FetchContent)
 
     FetchContent_Declare(

preview/sdl/DirectDisplaySdl.cpp

@@ -1,5 +1,4 @@
 #include "preview/sdl/DirectDisplaySdl.hpp"
-#include "preview/interfaces/Colour.hpp"
 #include "SDL3/SDL.h"
 #include "preview/interfaces/Colour.hpp"
 
@@ -8,7 +7,8 @@ namespace
     SDL_FRect Convert(infra::Region region)
     {
         SDL_FRect result;
-        SDL_RectToFRect(&SDL_Rect{ region.TopLeft().x, region.TopLeft().y, region.Width(), region.Height() }, &result);
+        SDL_Rect rect{ region.TopLeft().x, region.TopLeft().y, region.Width(), region.Height() };
+        SDL_RectToFRect(&rect, &result);
         return result;
     }
 }