shaderlib: Add alpha_blend function, extend place_texture example, update autocomplete - implements #23

shaderlib/transparency.gdshaderinc

@@ -0,0 +1,9 @@
+
+// Alpha Blending a over b after Bruce A. Wallace
+// sources:
+// - 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);
+}