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.