This refactors the iteration in restoration.c so that all the scary
stuff lies in a pair of general functions, filter_frame and
filter_rest_unit.

filter_frame is currently very simple, iterating over the restoration
units in the frame. Once we've made it so that restoration units don't
span tile boundaries, this function is the one we'll need to update to
iterate over tiles and then restoration units within the tile.

filter_rest_unit replaces the outer loop of the loop_*_filter_tile*
functions. It deals with chopping the restoration unit into stripes of
height procunit_height. When CONFIG_STRIPED_LOOP_RESTORATION is true,
it also deals with calling setup_processing_stripe_boundary and
restore_processing_stripe_boundary to use boundary data from the
deblocked output.

Some of the ugly #if/#endif blocks have been elided in the wiener
filter code (both low and high bit depth), by defining a convolve
alias based on USE_WIENER_HIGH_INTERMEDIATE_PRECISION.

There are also changes to extend const-ness for the source frame. I've
adopted the convention that the frame input is called "data" (as it
was before) while it's non-const. This is true as far as
filter_rest_unit. Then each "process one stripe" function takes a
const pointer to the source frame, at which point it's called "src".

The intention is that, once filter_rest_unit no longer needs a
RestorationInternal pointer, this function can be exposed in
restoration.h and can be used by pickrst.c

Change-Id: I18043a172ef0ca1154d87cf7f63e3a80944627cd