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>

.gitignore

@@ -83,6 +83,8 @@ VisualC/tests/testyuv/testyuv.bmp
 VisualC-GDK/**/Layout
 src/render/direct3d12/D3D12_*_One.h
 src/render/direct3d12/D3D12_*_Series.h
+src/gpu/d3d12/D3D12_*_One.h
+src/gpu/d3d12/D3D12_*_Series.h
 Directory.Build.props
 
 # for Android