Make D3D math functions static inline to reduce likelihood of linking conflicts

Convert SDL_d3dmath.c functions to static inline in SDL_d3dmath.h to make
it less likely to conflict when static linking SDL. raylib's SDL backend
does not work with a "normal" upstream binary static link. It has these
errors:

/usr/bin/ld: /usr/local/lib64/libSDL3.a(SDL_d3dmath.c.o): in function `MatrixIdentity':
SDL/src/render/SDL_d3dmath.c:35: multiple definition of `MatrixIdentity'; CMakeFiles/raylib.dir/rcore.c.o:rcore.c:(.text+0x18470): first defined here
/usr/bin/ld: /usr/local/lib64/libSDL3.a(SDL_d3dmath.c.o): in function `MatrixMultiply':
SDL/src/render/SDL_d3dmath.c:44: multiple definition of `MatrixMultiply'; CMakeFiles/raylib.dir/rcore.c.o:rcore.c:(.text+0x18540): first defined here
collect2: error: ld returned 1 exit status

It works if these functions aren't emitted, (i.e.
!SDL_VIDEO_RENDER_D3D(11|12|GPU|VULKAN)

Or, if SDL is not linked static. Which I know is preferred anyway.

In raylib's case, it doesn't use this code anyway so it's not a problem
to build it yourself with them disabled. But it's a minor hassle to be
incompatible with static linking libraries from upstream.

I can't see any good reason for these functions to not be static inline
already, so I just put them in the header and deleted the .c.

Raylib's conflicting case is a library with a public function exposed to
users. But SDL's case is not.

- Moved all matrix functions from SDL_d3dmath.c to SDL_d3dmath.h as static inline
- Removed SDL_d3dmath.c from all project files (Visual Studio and Xcode)
- Functions affected: MatrixIdentity, MatrixMultiply, MatrixScaling,
  MatrixTranslation, MatrixRotationX, MatrixRotationY, MatrixRotationZ

(cherry picked from commit 705ced8cf5)

VisualC-GDK/SDL/SDL.vcxproj

@@ -781,7 +781,6 @@
     <ClCompile Include="..\..\src\render\opengl\SDL_shaders_gl.c" />
     <ClCompile Include="..\..\src\render\opengles2\SDL_render_gles2.c" />
     <ClCompile Include="..\..\src\render\opengles2\SDL_shaders_gles2.c" />
-    <ClCompile Include="..\..\src\render\SDL_d3dmath.c" />
     <ClCompile Include="..\..\src\render\SDL_render.c" />
     <ClCompile Include="..\..\src\render\SDL_render_unsupported.c" />
     <ClCompile Include="..\..\src\render\SDL_yuv_sw.c" />

VisualC-GDK/SDL/SDL.vcxproj.filters

@@ -108,7 +108,6 @@
     <ClCompile Include="..\..\src\render\opengl\SDL_shaders_gl.c" />
     <ClCompile Include="..\..\src\render\opengles2\SDL_render_gles2.c" />
     <ClCompile Include="..\..\src\render\opengles2\SDL_shaders_gles2.c" />
-    <ClCompile Include="..\..\src\render\SDL_d3dmath.c" />
     <ClCompile Include="..\..\src\render\SDL_render.c" />
     <ClCompile Include="..\..\src\render\SDL_render_unsupported.c" />
     <ClCompile Include="..\..\src\render\SDL_yuv_sw.c" />