[mythtv] [PATCH] FilterManager cleanup, several new filters.

[Andrew Mahone]

This patch includes a fix for bufsize calculation in 
FilterManager::LoadFilters, and a rewrite of the format selection code.  The 
new loop iterates head to tail over the filter chain, instead of tail to 
head, and avoids multiple iterations over each filter's list of format 
conversions.  Handling of inpixfmt and outpixfmt set to FMT_NONE has been 
moved after the filter load loop, so that if a filter fails to init, they 
will be handled exactly as if no filters had been given.

The new filters are "convert", "forceyv12", "forceyuv422p", "forcergb24", and 
"forceargb32".  Convert currently supports conversion in both directions 
between YV12 and YUV422P, and is unoptimized, but runs at over 1k fps for 
480x480 on my system.  It is not intended to be used manually in a filter 
chain, but it will be inserted automatically as required by FilterManager.  I 
don't have plans to optimized these or provide other conversions right now, 
but I may do so if there appears to be demand for it.

The force* filters allow the user to manipulate FilterManager's filter 
selection.  For example, the filter string 
"forceyuv422p,kerneldeint,forceyv12,denoise3d" would cause capture to occur 
in YUV422P, which would be converted to YV12 after deinterlacing, and finally 
processed by denoise3d.

The force filters are also an example for a new filter feature that I put in 
several patches ago, which allows a filter to have itself removed from the 
chain, while allowing the rest of the chain to load.  This could also be used 
by a filter that decides at init time whether or not to load, such as a 
compute-intensive filter that could be configured to act as a no-op if the 
frame size is too large.  Because the no-op filter is removed from the chain 
before use, it causes no overhead, unless its presence forces a format 
conversion.

The filters README has been updated to document the no-op filter feature.
-- 
  Andrew Mahone
  andrewmahone AT eml DOT cc
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