obs-transitions: Add 'slide' transition

Closes jp9000/obs-studio#516

plugins/obs-transitions/easings.h

@@ -0,0 +1,11 @@
+#pragma once
+
+static inline float cubic_ease_in_out(float t)
+{
+	if (t < 0.5f) {
+		return 4.0f * t * t * t;
+	} else {
+		float temp = (2.0f * t - 2.0f);
+		return (t - 1.0f) * temp * temp + 1.0f;
+	}
+}