New player commands: decrease/increase tempo factor.

[AliceLR]

It would be nice to have player commands to increase/decrease the relative tempo factor. This requires updating to libxmp 4.7.0, so no milestone currently assigned. Proof-of-concept patch via libxmp/libxmp#966:

diff --git a/src/commands.c b/src/commands.c
index 9b4a8239..6266a402 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -145,6 +145,7 @@ static void change_sequence(xmp_context handle, const struct xmp_module_info *mi
  */
 void read_command(xmp_context handle, const struct xmp_module_info *mi, struct control *ctl)
 {
+	double factor;
 	int cmd;
 
 	cmd = read_key();
@@ -276,5 +277,23 @@ void read_command(xmp_context handle, const struct xmp_module_info *mi, struct c
 	case '<':
 		change_sequence(handle, mi, ctl, -1);
 		break;
+	case '[':
+		factor = xmp_get_tempo_factor_relative(handle);
+		if (factor >= 0.2) {
+			factor -= 0.1;
+			if (xmp_set_tempo_factor_relative(handle, factor) == 0) {
+				info_message("Time factor %.02f", factor);
+			}
+		}
+		break;
+	case ']':
+		factor = xmp_get_tempo_factor_relative(handle);
+		if (factor > 0.0) {
+			factor += 0.1;
+			if (xmp_set_tempo_factor_relative(handle, factor) == 0) {
+				info_message("Time factor %.02f", factor);
+			}
+		}
+		break;
 	}
 }