The non-H.26[45] codecs already use this form. Since we don't
currently generate I frames for codecs which support them separately
to IDR, the p_per_i variable is set to infinity by default so that it
doesn't interfere with any other calculation. (All the code for I
frames still exists, and it works for H.264 if set manually.)
This way they can be reused by other code without including the whole
decoder-specific hevcdec.h
Also, add the HEVC_ prefix to them, since similarly named values exist
for H.264 as well and are sometimes used in the same code.
This was not observed earlier because the only syntax element which
it normally misses with the current setup is slice_qp_delta, but that
is always going to be zero (in IDR frames QP isn't varied on the
slice) which will always exp-golomb code as a single 1 bit. The
immediately following part is the byte alignment, which is always a 1
bit followed by 0s which are ignored, so as long as the bitstream is
never aligned at that point we will never notice because the only
difference is that an ignored bit is a 1 instead of a 0.
This allows better checking of capabilities and will make it easier
to add more functionality later.
It also commonises some duplicated code around rate control setup
and adds more comments explaining the internals.
