Gstreamer backend reports that `video/mp4` mime type is not supported

[tharkum]

After implementation of supported selection of media element source from source children (PR servo/servo#39717) there was found that GStreamer backend is reporting that video/mp4 mime type is not supported while it is not true.

OK (video)
<video src="any-video-content.mp4">

OK (video + source without type)
<video><source src="any-video-content.mp4"></video>

FAIL (video + source with type)
<video><source src="any-video-content.mp4" type="video/mp4"></video>

See https://html.spec.whatwg.org/multipage/media.html#loading-the-media-resource:attr-source-type

Need to figure out reason why it happens (see backends/gstreamer/registry_scanner.rs)

Some associated WPT tests (canvas + video with wide gamut) on which it is reproduced with test expectation changes (mp4 vs webm):

• /html/canvas/element/manual/wide-gamut-canvas/canvas-display-p3-drawImage-video.html
• /html/canvas/element/manual/wide-gamut-canvas/canvas-display-p3-drawImage-ImageBitmap-video.html

NOTE: Some videos are encoded with H264 (High Profile) or H265 (Main 10 Profile) and both is decoding by Gstreamer eventually...
H264: html/canvas/element/manual/wide-gamut-canvas/resources/sRGB-FF0100.mp4
H265: html/canvas/element/manual/wide-gamut-canvas/resources/Rec2020-222000000.mp4

On local PC gst-inspect-1.0 reports the following plugins (ubuntu22.04)

$ gst-inspect-1.0 | grep h264
libav:  avdec_h264: libav H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 decoder
libav:  avenc_h264_omx: libav OpenMAX IL H.264 video encoder encoder
openh264:  openh264dec: OpenH264 video decoder
openh264:  openh264enc: OpenH264 video encoder
rtp:  rtph264depay: RTP H264 depayloader
rtp:  rtph264pay: RTP H264 payloader
typefindfunctions: video/x-h264: h264, x264, 264
uvch264:  uvch264deviceprovider (GstDeviceProviderFactory)
uvch264:  uvch264mjpgdemux: UVC H264 MJPG Demuxer
uvch264:  uvch264src: UVC H264 Source
va:  vah264dec: VA-API H.264 Decoder
videoparsersbad:  h264parse: H.264 parser

$ gst-inspect-1.0 | grep h265
libav:  avdec_h265: libav HEVC (High Efficiency Video Coding) decoder
rtp:  rtph265depay: RTP H265 depayloader
rtp:  rtph265pay: RTP H265 payloader
typefindfunctions: video/x-h265: h265, x265, 265
va:  vah265dec: VA-API H.265 Decoder
videoparsersbad:  h265parse: H.265 parser