iOS: Add "allowTranscoding" option to Ti.Media.openPhotoGallery()

Summary:
When selecting a video from the gallery, iOS will transcode/convert the selected video by default and return the newly formatted video instead. This can cause a very long delay if the video is large.

Solution:
Add an iOS-only "allowTranscoding" option for our Ti.Media.openPhotoGallery() method, when set false, will return the selected video as-is without transcoding it. This will solve the long delay issue.

Ti.Media.openPhotoGallery({

	allowTranscoding: false,

	mediaTypes: [Ti.Media.MEDIA_TYPE_VIDEO],

	success: function(e) {

		// Got it.

	},

});

Note:
There is no equivalent to this on Android, which always returns a reference to the video as-is. So, this is an iOS only feature.

Native Implementation Details:
Add this code at line 1746. I can provide a PR

if ([TiUtils isIOS11OrGreater]) {

      BOOL disableVideoCompression = [TiUtils boolValue:@"disableVideoCompression" properties:args def:NO];

      if (disableVideoCompression) {

        picker.videoExportPreset = AVAssetExportPresetPassthrough;

      }

    }

    [self displayModalPicker:picker settings:args];