Fix device capability checking in v4l2 input.

The capabilities flags that were used previously describe all
capabilities the physical device offers. This would cause devices
that are accessible through multiple device nodes to show up with
all device nodes while only one of the nodes might actually offer
the needed video capture capability.

If the device has more nodes the CAP_DEVICES_CAP flag might be set
in which case the device_caps field is filled with the capabilities
that only apply to that specific node that is opened.
Do not explicitly set a default device in v4l2 input.

This was added at a time where the source properties dialog did not
pop up automatically on source creation. Now when the properties are
displayed the first device in the select input will be selected by
default if there was none already specified by the source settings.

This will make the code cleaner and also save one redundant round of
device enumeration.
Use helper function to set input in v4l2 input.
Use helper function to set format in v4l2 input.

Since the helper function also needs to pack/unpack the resolution, the
pack/unpack functions were moved to the helper library and prefixed with
v4l2_ in order to avoid possible collisions.
Use helper function to set framerate in v4l2 input.
Add option to leave settings unchanged in v4l2 input.

This adds an additional option for all settings to leave the current
configuration of the device unchanged.
Minor refactoring in v4l2 input.

We don't really need to use a dstr here, simply using the
right format specifier for blog will suffice.
Refactor update function for v4l2 input.

The old implementation of this function assumed that there would be some
settings that could be changed on the fly without restarting the
capture. That was actually never used for any setting.
Refactor source structure in v4l2 input.

Due to the refactoring of the update function the separation of data
members only to be accessed from inside/outside the capture thread is
no longer needed.
Add video format check in v4l2 input.

This adds a check whether the video format from the device is compatible
with obs. This could either happen if the "Leave unchanged" option is
selected for the video format, or if the driver simply overwrites the
requested video format.
Merge pull request #266 from fryshorts/v4l2-input

Fix device capabilities checking and add "Leave unchanged" option for all settings.