Make the pixel formats which were defined as macros:

PIX_FMT_ARGB
PIX_FMT_RGBA
PIX_FMT_ABGR
PIX_FMT_BGRA

defined as enum PixelFormat values, and viceversa make:
PIX_FMT_RGB32
PIX_FMT_RGB32_1
PIX_FMT_BGR32
PIX_FMT_BGR32_1

defined as macros, also resort accordingly the enum PixelFormat
list.
Also make avcodec_get_pix_fmt() recognize the "rgb32" and "bgr32"
aliases, in order to make ffmpeg pass regressions test.

This change breaks ABI backward compatibility.

Originally committed as revision 18163 to svn://svn.ffmpeg.org/ffmpeg/trunk