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>

test/CMakeLists.txt

@@ -401,6 +401,7 @@ add_sdl_test_executable(testcontroller TESTUTILS SOURCES testcontroller.c gamepa
 add_sdl_test_executable(testgeometry TESTUTILS SOURCES testgeometry.c)
 add_sdl_test_executable(testgl SOURCES testgl.c)
 add_sdl_test_executable(testgles SOURCES testgles.c)
+add_sdl_test_executable(testgpu_spinning_cube SOURCES testgpu_spinning_cube.c)
 if(ANDROID)
     target_link_libraries(testgles PRIVATE GLESv1_CM)
 elseif(IOS OR TVOS)