vo_opengl: remove redundant GL state initialization

All of these are already the defaults.

One exception is glDepthMask(), which is enabled by default. But if the
framebuffer has no depth buffer anyway, it shouldn't make a difference.
audio: allow more than 20 channel map entries

This could trigger an assertion when using ao_alsa or ao_coreaudio. The
code was simply assuming the number of channel maps was bounded
statically (which was true at first in both AOs).

Fix by using dynamic memory allocation. It needs to be explicitly
enabled by the AOs by setting a temp context, because otherwise the
memory couldn't be freed. (Or at least this seems to be the most elegant
solution.)

Fixes #1306.
options: simplify channel map validation

The min=-1 case was never used, and the channel map must always be valid
(or in some cases, invalid but empty).
options: fix swapped deprecated/replacement in warning

This was relevant for --af and --vo (both use this option type, and have
deprecated aliases).