General housekeeping: Make plugin buffer functions take size_t * instead of int ...

General housekeeping: Make plugin buffer functions take size_t * instead of int * to match the parameter type of the buffer functions called in the core. Get rid of unsafe int * <==> size_t * casting. Use ssize_t where int was used and size_t where unsigned int was used in the buffer calls to not alter signedness in the plugins. No API version change since it should only be an issue for 64-bit sim builds.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@13233 a1c6a512-1295-4272-9138-f99709370657