gst/adder/: Cleanup variable names to make the adder-loop easier to understand.

gst/adder/: Cleanup variable names to make the adder-loop easier to understand.

Original commit message from CVS:
* gst/adder/Makefile.am:
* gst/adder/gstadder.c:
Cleanup variable names to make the adder-loop easier to understand.
Also try to use liboil to spee it up, but ifdef it out as it does not
make any change for me (Intel pentim M (sse,sse2) please try on other
systems).