spa: bump channels to 128 again

Remove the SPA_AUDIO_MAX_POSITION define and use the
SPA_AUDIO_MAX_CHANNELS again.

Make a compile time define to override the default max channels of 64.

Make sure we compile the SPA library with the default 64 channels. If
you use the SPA library on a spa_audio_info you will get 64 channel
support, like before. If you want more channels, you will need to make
a padded structure or redefine the MAX_CHANNELS before you use the
spa_audio_info structures. You can use the padded structure with the
new functions that take the structure size.

With the extra checks in the parsing code, we avoid making a
valid spa_audio_info with too many channels that don't fit in the
structure. This means that code that receives a spa_audio_info can
assume there is enough padding for all the channels.

meson.build

@@ -115,6 +115,7 @@ cc_flags = common_flags + [
   '-Werror=old-style-definition',
   '-Werror=missing-parameter-type',
   '-Werror=strict-prototypes',
+  '-DSPA_AUDIO_MAX_CHANNELS=128u',
 ]
 add_project_arguments(cc.get_supported_arguments(cc_flags), language: 'c')