mp_common: add MPMAX/MPMIN macros

From now on, usage of these macros is encouraged over using FFMAX and
FFMIN. FFMAX and FFMIN are perfectly fine, and the added macros are
actually exactly the same as the FFMAX and FFMIN definitions. But they
require including libavutil headers, and certain differences between
Libav and FFmpeg very often introduced breakages if these macros were
somehow not defined because a header was not recursively included.
Defining this macro on our own is the best way to escape from this
annoying issue.
w32_common: don't use aspdat.asp for aspect

aspdat.asp is a problem, because it's updated when the VO calls
vo_get_src_dst_rects(). Nothing guarantees that the value has been
updated when the w32 code accesses it.

Instead, use the aspect vo_w32_config() was called with.
README: copy paragraph about --enable-* from configure

This issue hits users way too often. Copy the explanation printed by the
configure script to the README to give it more visibility.

We will fix this properly once we have a new build system.