Restructured shaderlib by moving functions and renaming files

shaderlib/common.gdshaderinc

@@ -3,3 +3,13 @@
 float cbrt(float x) {
 	return pow(x, 1.0/3.0);
 }
+
+/*
+	Alpha Blending a over b after Bruce A. Wallace
+	source: https://en.wikipedia.org/wiki/Alpha_compositing
+*/
+vec4 alpha_blend(vec4 b, vec4 a) {
+	float alpha = a.a + (b.a * (1.0 - a.a));
+	vec3 col = ((a.rgb*a.a) + ((b.rgb*b.a) * (1.0 - a.a)) / alpha);
+	return vec4(col.r, col.g, col.b, alpha);
+}