Fix photo resolution, recording quality, and blank preview for raw-only cameras

[PlugMyLife]

What this does

Three small fixes to CameraView.vala (20 lines total), each independent:

1. Consider raw caps when picking photo resolution. Photo sizing only
   inspected image/jpeg capabilities. A camera that exposes only raw
   formats always fell through to the 640×480 default, and photos from a
   16:9-only sensor were stretched to 4:3. Now video/x-raw structures
   are scanned too, so photos use the sensor's native resolution.

2. Configure vp8enc for realtime capture. The encoder was created
   with its defaults (256 kbps target, best-quality/slowest deadline),
   which can't encode an HD stream in real time — the encoder falls
   behind, the leaky preview queue drops frames, and recordings come out
   choppy at ~80 kbps for 720p30. This sets the realtime deadline,
   spreads work across the available CPU cores, and scales the target
   bitrate with frame size (~3.7 Mbps at 720p).

3. Handle a missing gtk4paintablesink. Every element in the
   recording path reports a missing-plugin message, but the preview sink
   was used unchecked. Without the GTK4 GStreamer plugin the preview
   silently stayed black while photos and recording kept working. This
   adds the same missing-plugin handling and surfaces the existing
   "Unable To View Camera" dialog.

Why

I hit all three on a 2015 MacBook Pro (MacBookPro11,4), whose FaceTime HD
webcam is a PCIe device driven by the out-of-tree
facetimehd driver and exposes
only raw (non-JPEG) formats. The bugs are general, though — any raw-only
camera hits #1 and #2, and #3 affects any host missing the GTK4 sink.

Before / after (measured on that machine)

	before	after
photo resolution	640×480, wrong aspect	1280×720, correct
recording bitrate (720p30)	~80 kbps	~2 Mbps
recording smoothness	drops frames	smooth
preview, GTK4 sink missing	silent black screen	clear error dialog

Testing

Built against current main (meson + ninja, compiles clean) and run
on the hardware above: photos now save at native resolution, recordings
are smooth, and removing the GTK4 sink produces the error dialog instead
of a black screen.

[stsdc]

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