[GStreamer] Drop unused m_do_pause_is_null attribute

Since "[GStreamer] Set the state to NULL when the stream is paused" has
been reverted the m_do_pause_is_null attribute is not necessary anymore.
It was there to temporary deactivate the NULL-on-PAUSED behaviour to
avoid conflicts when reconfiguring the pipeline. Now the pipeline goes
through the PAUSED state when reconfiguring.
[GStreamer] Make sure to take into account the initial target size

Sometimes we fail to detect the correct size and thus the video plays
slower and look scaled (eg.  https://www.youtube.com/watch?v=n77cwRJUrLg).

`perf report` would then show that a lot of time is spent in
pixman_scaled_nearest_scanline_0565_0565_SRC_asm_armv6().
[GStreamer] Set WEBKIT_DEBUG_VIDEO_SIZE to print video size information

As it's critical to avoid any software video scaling, this kind of
information may help to debug such cases.
[DirectPaint] Allow filling areas that are very small compared to the video rectangle

Add GraphicsContext::setSkipThreshold() to set a threshold under which
small areas are allowed to be filled with fillRect(), eg. custom video
controls.
[TBS] Add hasTiles public method to check whether there are tiles available.

This method will be used on the GTK+ webkit1 webview when deciding whether to render
the view from the Tiled Backing Store.
webkitwebview: Only render view from TBS if there are tiles available

The checker pattern is painted by TiledBackingStore::paint (invoked when rendering the view)
when a tile is not found.
With this patch, TiledBackingStore::paint is only invoked when rendering the view if TBS has tiles
available.

This avoids displaying the checker pattern when loading pages, when no tiles are available.

It also avoids displaying the checker pattern when the horizontal allocation of the view changes,
which without this patch happens because when the horizontal allocation changes, the view updates
the TBS tile size (with TiledBackingStore::setTileSize) to match the new allocation width, which
causes TBS to destroy all tiles, and then the view is rendered again thus calling
TiledBackingStore::paint with no tiles available.
[SharedTimerGtk] set a lower priority than the gstreamer player

Since 3a0830257566e6206878cb0eb460653af3ac3c6f JS had a higher
priority than the video so it causes frame dropping.

See RPI-327, RPI-329 and RPI-349
[ChromeClientGtk] Update TBS tiles when notified of contents size changes.

Updating tiles is required here because when the webview is resized,
although the FrameView is also resized, the FrameView contents size is still outdated,
thus invoking TiledBackingStore::coverWithTilesIfNeeded from inside the
webview resize handler would have no effect as the method checks, among others, if the
FrameView content size (retrieved with Frame::tiledBackingStoreContentsRect) changed when
deciding whether tiles should be updated.

We can safely invoke TiledBackingStore::coverWithTilesIfNeeded from
ChromeClient::contentsSizeChanged as this method is only invoked after the FrameView
content size is already updated, making sure the TBS uses the proper contents size when
checking if a tile update is needed.