Add the SDL_GPU API

Project Lead: Evan Hemsley <evan@moonside.games>

Co-designer, Metal Port, Console Ports:

Co-authored-by: Caleb Cornett <caleb.cornett@outlook.com>

Production, QA, Debug:

Co-authored-by: Ethan Lee <flibitijibibo@gmail.com>

SDL_Render Driver, Bugfixes:

Co-authored-by: Andrei Alexeyev <akari@taisei-project.org>

Additional D3D12 Programming, Bugfixes:

Co-authored-by: Bart van der Werf <bluelive@gmail.com>

Bugfixes and Feedback:

Co-authored-by: Zakary Strange <zakarystrange@gmail.com>
Co-authored-by: meyraud705 <meyraud705@gmail.com>
Co-authored-by: Joshua T. Fisher <playmer@gmail.com>
Co-authored-by: Topi Ritala <ritalat@fastmail.com>
Co-authored-by: David Gow <david@ingeniumdigital.com>

Original API Proposal:

Co-authored-by: Ryan C. Gordon <icculus@icculus.org>

VisualC-WinRT/SDL-UWP.vcxproj

@@ -125,6 +125,7 @@
     <ClInclude Include="..\src\events\SDL_touch_c.h" />
     <ClInclude Include="..\src\events\SDL_windowevents_c.h" />
     <ClInclude Include="..\src\filesystem\SDL_sysfilesystem.h" />
+    <ClInclude Include="..\src\gpu\SDL_sysgpu.h" />
     <ClInclude Include="..\src\haptic\SDL_haptic_c.h" />
     <ClInclude Include="..\src\haptic\SDL_syshaptic.h" />
     <ClInclude Include="..\src\haptic\windows\SDL_dinputhaptic_c.h" />
@@ -343,6 +344,9 @@
     <ClCompile Include="..\src\file\SDL_iostream.c" />
     <ClCompile Include="..\src\filesystem\SDL_filesystem.c" />
     <ClCompile Include="..\src\filesystem\windows\SDL_sysfsops.c" />
+    <ClCompile Include="..\src\gpu\SDL_gpu.c "/>
+    <ClCompile Include="..\src\gpu\d3d11\SDL_gpu_d3d11.c "/>
+    <ClCompile Include="..\src\gpu\d3d12\SDL_gpu_d3d12.c "/>
     <ClCompile Include="..\src\haptic\dummy\SDL_syshaptic.c" />
     <ClCompile Include="..\src\haptic\SDL_haptic.c" />
     <ClCompile Include="..\src\haptic\windows\SDL_dinputhaptic.c" />