Turn on error logging at all times

Turn on error logging at all times

While releasing 0.29.2 the distcheck run produced a number of error
messages that had to be fixed in 349015e1fc5d912ba4253133b90e751d0b.
These were not caught before so nobody had actually run pixman with
debugging turned on. It's not the first time this has happened, see
5b0563f39eb29e4ae431717696174da5 for example.

So this patch makes the return_if_fail() macros use unlikely() around
the expressions and then turns on error logging at all times. The
performance hit should negligible since we were already evaluating the
expressions.

The place where DEBUG actually does cause a performance hit is in the
region selfcheck code, and that will still only be enabled in
development snapshots.