Add multiple missing pointer overloads

THis adds some missing overloads that Ryujinx uses.

Changelog:
- Add a new overload of SDL_MixAudioFormat to allow passing raw pointers
  as dest and src (useful for usage with SDL_AudioSpec's callback).
- Add a new overload of SDL_OpenAudioDevice to allow passing null for
  the device parameter.
- Add a new overload of SDL_Vulkan_GetInstanceExtensions to allow
  passing null for the pName parameter.

src/SDL2.cs

@@ -2277,6 +2277,17 @@ namespace SDL2
 		[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
 		public static extern void SDL_Vulkan_UnloadLibrary();
 
+		/* window refers to an SDL_Window*, pNames to a const char**.
+		 * Only available in 2.0.6 or higher.
+		 * This overload allows for IntPtr.Zero (null) to be passed for pNames.
+		 */
+		[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
+		public static extern SDL_bool SDL_Vulkan_GetInstanceExtensions(
+			IntPtr window,
+			out uint pCount,
+			IntPtr pNames
+		);
+
 		/* window refers to an SDL_Window*, pNames to a const char**.
 		 * Only available in 2.0.6 or higher.
 		 */
@@ -7777,6 +7788,17 @@ namespace SDL2
 			int volume
 		);
 
+		/* format refers to an SDL_AudioFormat */
+		/* This overload allows raw pointers to be passed for dst and src. */
+		[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
+		public static extern void SDL_MixAudioFormat(
+			IntPtr dst,
+			IntPtr src,
+			ushort format,
+			uint len,
+			int volume
+		);
+
 		/* format refers to an SDL_AudioFormat */
 		[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
 		public static extern void SDL_MixAudioFormat(
@@ -7801,6 +7823,17 @@ namespace SDL2
 			IntPtr obtained
 		);
 
+		/* uint refers to an SDL_AudioDeviceID */
+		/* This overload allows for IntPtr.Zero (null) to be passed for device. */
+		[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
+		public static extern unsafe uint SDL_OpenAudioDevice(
+			IntPtr device,
+			int iscapture,
+			ref SDL_AudioSpec desired,
+			out SDL_AudioSpec obtained,
+			int allowed_changes
+		);
+
 		/* uint refers to an SDL_AudioDeviceID */
 		[DllImport(nativeLibName, EntryPoint = "SDL_OpenAudioDevice", CallingConvention = CallingConvention.Cdecl)]
 		private static extern unsafe uint INTERNAL_SDL_OpenAudioDevice(