[FFmpeg] version bump to n1.2 (rev e820e3a)

FFmpeg version bump to n1.2 (rev e820e3a). 

This solves a bunch of more or less serious problems that we currently struggle with in version 0.10.2, for instance preventing users to watch certain content types, hangs or causes stuttering video playback. Also, n1.2 is the last ffmpeg version that supports the old api. Essentially, this upgrade contains the following files and directories:

- dir: lib/ffmpeg
FFmpeg n1.2 fork (rev e820e3a) including all necessary xbmc add on patches.
 
- dir: lib/ffmpeg/patches
This directory contains all the XBMC add-on patches applied to ffmpeg n1.2.

- file: lib/ffmpeg/patches/README-patches
Brief description of the xbmc add on patches.

- file: lib/ffmpeg/ffmepg/patches/obsolete-patches
Contains obsolete xbmc custom patches that were NOT applied to n1.2.

- file: lib/ffmpeg/build_xbmc_win32.sh
Win32 build script that generates the include file DllPaths_generated_win32_ffmpeg.h and is called from buildmingwlibs.sh.

- file: lib/DllAvFormat.h
Deals with the decrepated version av_read_frame_flush -> ff_read_frame_flush

- file: lib/Makefile.in
When ffmpeg is configured for osx then remove duplicate log2_tab.o from libavcodec.a, libavformat.a and libswresample.a

- file: xbmc/cores/dvdplayer/DVDCodecs/Video/DVDVideoCodecFFmpeg.cpp & xbmc/cores/dvdplayer/DVDCodecs/Video/DVDVideoCodecFFmpeg.h
Resolves dllAvFilter dependencies of LibPostProc that previously required LD_LIBRARY_PATH

- General adjustments to n1.2 (files)
file: xbmc/cores/dvdplayer/DVDCodecs/Video/DVDVideoCodecLibMpeg2.cpp
file: xbmc/cores/dvdplayer/DVDCodecs/Video/DXVA.cpp
file: xbmc/cores/dvdplayer/DVDCodecs/DVDCodecs.h
file: xbmc/cores/dvdplayer/DVDAudio.h
file: xbmc/cores/dvdplayer/DVDDemux.h
file: xbmc/cores/dvdplayer/DVDPlayerAudio.h
file: xbmc/cores/dvdplayer/DVDStreamInfo.h

- file: tools/depends/native/gas-preprocessor-native/gas-preprocessor.pl
Updated version that manages libavcodec/arm assembler optimizations. Source: github.com/mansr/gas-preprocessor, rev 76a72f00e (Dec 03, 2012).

- file: project/Win32BuildSetup/BuildSetup.bat
Win32 build script adjusted to ffmpeg n1.2

- file: project/Win32BuildSetup/buildmingwlibs.sh
Win32 build script that generates the include file DllPaths_generated_win32_ffmpeg.h.

- file: xbmc/DllPaths_win32.h
DllPaths_win32.h is now modified to include the auto-genrerated file "DllPaths_generated_win32_ffmpeg.h".