Windows: download SDL3 on demand instead of borrowing from ObjectivelyMVC

The Windows VS build sourced its SDL3 headers and libs from
ObjectivelyMVC's bundled libs/sdl, inverting the dependency graph
(MVC sits on top of GPU, not beneath it).

Add sdl3.targets, which downloads the official SDL3 VC development
package from libsdl.org on demand and caches it under libs/. CI and
local developers now share a single code path, and GPU owns its SDL3
core dependency directly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jdolan

.gitignore

@@ -18,6 +18,7 @@ Examples/HelloCompute
 Frameworks/
 Makefile
 Makefile.in
+ObjectivelyGPU.vs15/libs/
 aclocal.m4
 autom4te.cache/
 build/

ObjectivelyGPU.vs15/ObjectivelyGPU.vcxproj

@@ -69,9 +69,9 @@
   <PropertyGroup>
     <OutDir>bin\$(Platform)$(Configuration)\</OutDir>
     <IntDir>tmp\$(ProjectName)\$(Platform)$(Configuration)\</IntDir>
-    <!-- SDL3 headers come from ObjectivelyMVC's bundled libs (repos are checked out side-by-side) -->
-    <IncludePath>Sources\;..\Sources\;..\..\Objectively\Sources\;..\..\Objectively\Objectively.vs15\Sources\;..\..\ObjectivelyMVC\ObjectivelyMVC.vs15\libs\sdl\;$(IncludePath)</IncludePath>
-    <LibraryPath>..\..\Objectively\Objectively.vs15\bin\$(Platform)$(Configuration)\;..\..\ObjectivelyMVC\ObjectivelyMVC.vs15\libs\sdl\lib\$(Platform)\;$(LibraryPath)</LibraryPath>
+    <!-- SDL3 search paths are provided by sdl3.targets, which downloads SDL3 on demand. -->
+    <IncludePath>Sources\;..\Sources\;..\..\Objectively\Sources\;..\..\Objectively\Objectively.vs15\Sources\;$(IncludePath)</IncludePath>
+    <LibraryPath>..\..\Objectively\Objectively.vs15\bin\$(Platform)$(Configuration)\;$(LibraryPath)</LibraryPath>
     <UseMultiToolTask>true</UseMultiToolTask>
   </PropertyGroup>
   <ItemDefinitionGroup>
@@ -108,6 +108,7 @@
     </Link>
   </ItemDefinitionGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+  <Import Project="sdl3.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
 </Project>

ObjectivelyGPU.vs15/sdl3.targets

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <!--
+    Fetches the official SDL3 VC development libraries from libsdl.org and wires
+    them into the compiler and linker search paths.  This mirrors what CI does,
+    so local developers and CI share a single code path: the libraries are
+    downloaded once, on demand, and cached under libs/ (which is .gitignored).
+    Bump SDL3Version to upgrade; delete libs/ to force a re-download.
+  -->
+  <PropertyGroup>
+    <SDL3Version>3.4.2</SDL3Version>
+    <SDL3Dir>$(MSBuildThisFileDirectory)libs\SDL3-$(SDL3Version)\</SDL3Dir>
+    <SDL3LibArch Condition="'$(Platform)' == 'Win32'">x86</SDL3LibArch>
+    <SDL3LibArch Condition="'$(SDL3LibArch)' == ''">x64</SDL3LibArch>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <IncludePath>$(SDL3Dir)include\;$(IncludePath)</IncludePath>
+    <LibraryPath>$(SDL3Dir)lib\$(SDL3LibArch)\;$(LibraryPath)</LibraryPath>
+  </PropertyGroup>
+
+  <Target Name="DownloadSDL3"
+          BeforeTargets="ClCompile"
+          Condition="!Exists('$(SDL3Dir)include\SDL3\SDL.h')">
+    <Message Importance="high" Text="Downloading SDL3 $(SDL3Version) development libraries..." />
+    <Exec WorkingDirectory="$(MSBuildThisFileDirectory)"
+          Command="powershell -NoProfile -ExecutionPolicy Bypass -Command &quot;$ErrorActionPreference = 'Stop'; New-Item -ItemType Directory -Force libs | Out-Null; Invoke-WebRequest -Uri 'https://github.com/libsdl-org/SDL/releases/download/release-$(SDL3Version)/SDL3-devel-$(SDL3Version)-VC.zip' -OutFile 'libs\sdl3.zip'; Expand-Archive -Force 'libs\sdl3.zip' 'libs'; Remove-Item 'libs\sdl3.zip'&quot;" />
+  </Target>
+
+</Project>