Added SDL_GetTextureDXGIResource() to get the DXGI resource associated with a render texture.

Also switched the D3D11 and D3D12 renderers to use real NV12 textures for NV12 data.

The combination of these two changes allows us to implement 0-copy video decode and playback for D3D11 in testffmpeg without any access to the renderer internals.

src/dynapi/SDL_dynapi.sym

@@ -907,6 +907,7 @@ SDL3_0.0.0 {
     SDL_SetAudioStreamFrequencyRatio;
     SDL_SetAudioPostmixCallback;
     SDL_GetAudioStreamQueued;
+    SDL_GetTextureDXGIResource;
     # extra symbols go here (don't modify this line)
   local: *;
 };

src/dynapi/SDL_dynapi_overrides.h

@@ -932,3 +932,4 @@
 #define SDL_SetAudioStreamFrequencyRatio SDL_SetAudioStreamFrequencyRatio_REAL
 #define SDL_SetAudioPostmixCallback SDL_SetAudioPostmixCallback_REAL
 #define SDL_GetAudioStreamQueued SDL_GetAudioStreamQueued_REAL
+#define SDL_GetTextureDXGIResource SDL_GetTextureDXGIResource_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -978,3 +978,4 @@ SDL_DYNAPI_PROC(float,SDL_GetAudioStreamFrequencyRatio,(SDL_AudioStream *a),(a),
 SDL_DYNAPI_PROC(int,SDL_SetAudioStreamFrequencyRatio,(SDL_AudioStream *a, float b),(a,b),return)
 SDL_DYNAPI_PROC(int,SDL_SetAudioPostmixCallback,(SDL_AudioDeviceID a, SDL_AudioPostmixCallback b, void *c),(a,b,c),return)
 SDL_DYNAPI_PROC(int,SDL_GetAudioStreamQueued,(SDL_AudioStream *a),(a),return)
+SDL_DYNAPI_PROC(IDXGIResource*,SDL_GetTextureDXGIResource,(SDL_Texture *a),(a),return)