Improve EFFECT_VWAVE_CROSSFADE

Improve the math in this effect so that it matches Alicesoft's
implementation. This is almost a pixel-perfect recreation of the effect.
The only difference is that our waves are rendered with a smoother edge.

shaders/effects/vwave_crossfade.f.glsl

@@ -22,8 +22,21 @@ uniform float progress;  // effect progress (0..1)
 in vec2 tex_coord;
 out vec4 frag_color;
 
-vec3 get_pixel(vec2 xy) {
-        return mix(texture(old, xy).rgb, texture(tex, xy).rgb, progress);
+#define M_PI 3.1415926535897932384626433832795
+#define OLD_PERIOD ((2.0 * M_PI) / 360.0)
+#define NEW_PERIOD ((2.0 * M_PI) / 720.0)
+#define OLD_SHIFT (-M_PI)
+#define NEW_SHIFT (M_PI / 2.0)
+
+vec3 get_pixel(vec2 old_xy, vec2 new_xy) {
+	// influence of old color is described by: C_old = (1-t)^2
+	// influence of new color increases linearly: C_new = t
+	// the resulting blend is not normalized
+	vec3 old = texture(old, old_xy).rgb;
+	vec3 new = texture(tex, new_xy).rgb;
+	float old_weight = (1.0 - progress) * (1.0 - progress);
+	float new_weight = progress;
+	return (old * old_weight + new * new_weight);
 }
 
 float triangle(float f) {
@@ -31,9 +44,14 @@ float triangle(float f) {
 }
 
 void main() {
+	// old: x = amp * sin((2pi/360)*y + phase + -pi)
+	// new: x = amp * sin((2pi/720)*y + phase + pi/2)
         vec2 p = tex_coord * resolution;
-        float scale = triangle(progress) * 80.0; // (0..0.5..1) -> (0..80..0);
-        float wobble = progress * 24.0;
-        float offset = sin((p.y / 100.0) - wobble) * scale;
-        frag_color = vec4(get_pixel(fract(vec2(p.x + offset, p.y) / resolution)), 1.0);
+        float amp = triangle(progress) * 80.0; // (0..0.5..1) -> (0..80..0);
+	float phase = progress * M_PI * 8.0;
+	float old_off = amp * sin(OLD_PERIOD * p.y + OLD_SHIFT + phase);
+	float new_off = amp * sin(NEW_PERIOD * p.y + NEW_SHIFT + phase);
+	vec2 old_xy = fract(vec2(p.x + old_off, p.y) / resolution);
+	vec2 new_xy = fract(vec2(p.x - new_off, p.y) / resolution);
+        frag_color = vec4(get_pixel(old_xy, new_xy), 1.0);
 }