Fix channel panning ramps.

It looks like in FT2, panning is made using an exponential scale, based on sqrt. This PR allows to match panning performed by MilkyTracker and OpenMPT. MilkyTracker: https://github.com/milkytracker/MilkyTracker/blob/be016986168a4d03b8d160303fa61296070a6e2c/src/milkyplay/ChannelMixer.cpp#L583 FT2-clone: https://github.com/8bitbubsy/ft2-clone/blob/8af2e42736b61a36a0d54bede93a47a1bab35f5d/src/ft2_audio.c#L192 The attached test makes it easy to reproduce the issue. Co-developed with @bryc who also prepared the test.
Artefact2 · Apr 13, 2021 · ad599b8 · ad599b8
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commit ad599b8
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diff --git a/src/play.c b/src/play.c
@@ -1362,8 +1362,8 @@ static void xm_sample(xm_context_t* ctx, float* left, float* right) {
 		const float fval = xm_next_of_sample(ch);
 
 		if(!ch->muted && !ch->instrument->muted) {
-			*left += fval * ch->actual_volume * (1.f - ch->actual_panning);
-			*right += fval * ch->actual_volume * ch->actual_panning;
+			*left += fval * ch->actual_volume * sqrt(1.f - ch->actual_panning);
+			*right += fval * ch->actual_volume * sqrt(ch->actual_panning);
 		}
 
 #if XM_RAMPING

diff --git a/tests/panning-law.xm b/tests/panning-law.xm