Update macOS GitHub runner, SDL and extend compatibility (#217)

* Update SDL and SDL_mixer on macOS

* Extend (Intel) Mac compatibility all the way to OS X El Capitan

* Update macOS runner due to GitHub deprecation but still use and Xcode version that supports 10.11

Platform/macOS/Info.plist

@@ -28,6 +28,8 @@
 	</array>
 	<key>CSResourcesFileMapped</key>
 	<true/>
+	<key>LSMinimumSystemVersion</key>
+	<string>10.11</string>
 	<key>NSHighResolutionCapable</key>
 	<true/>
 </dict>