fixed-point resample: remove 1-bit shift right before interpolation

This was added in
https://gitlab.xiph.org/xiph/speexdsp/-/commit/0dd7bfebe55abcac7e9acbca9d2ac2eddeee2b6f#5424d940076d3b85a3fc2d9f2dcf0d905c43abf0_466_472
to address truncation reported at
http://lists.xiph.org/pipermail/speex-dev/2009-June/007277.html

Truncation of the most-signficant bit was occuring on conversion to
spx_word16_t in MULT16_32_Q15().  Changes to MULT16_32_Q15() mean this will no
longer occur.

The associated adjustment to the bit-shift before saturation was accidentally
removed in
https://gitlab.xiph.org/xiph/speexdsp/-/commit/0e5d424fdba2fd1c132428da38add0c0845b4178#6479ffd77de750bc70cf92af3f9b8a4c1e15a98a_473_478
which caused the output to have half the intended amplitude when the
interpolating resampler was used.

Reported by Andreas Pehrson.

Author: karlt

libspeexdsp/resample.c

@@ -473,7 +473,7 @@ static int resampler_basic_interpolate_single(SpeexResamplerState *st, spx_uint3
       }
 
       cubic_coef(frac, interp);
-      sum = MULT16_32_Q15(interp[0],SHR32(accum[0], 1)) + MULT16_32_Q15(interp[1],SHR32(accum[1], 1)) + MULT16_32_Q15(interp[2],SHR32(accum[2], 1)) + MULT16_32_Q15(interp[3],SHR32(accum[3], 1));
+      sum = MULT16_32_Q15(interp[0],accum[0]) + MULT16_32_Q15(interp[1],accum[1]) + MULT16_32_Q15(interp[2],accum[2]) + MULT16_32_Q15(interp[3],accum[3]);
       sum = SATURATE32PSHR(sum, 15, 32767);
 #else
       cubic_coef(frac, interp);