shaderlib: Implement place_texture() and provide an example, replace mix.gdshader example; implements #17

examples/place_texture.gdshader

@@ -0,0 +1,14 @@
+shader_type canvas_item;
+
+#include "res://shaderlib/transform.gdshaderinc"
+
+//!load ./swamp.jpg
+//!load+ img2 ./grass.png
+
+uniform sampler2D img2: repeat_disable, filter_nearest;
+
+void fragment() {
+	vec4 grass = place_texture(img2, UV, TEXTURE_PIXEL_SIZE, vec2(0, .47), vec2(1));
+	grass.rgb += (vec3(0.03, 0.07, 0.11) - ((UV.y - .8) * 0.15)); // color correction
+	COLOR.rgb = mix(COLOR.rgb, grass.rgb, grass.a);
+}