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

shaderlib/transform.gdshaderinc

@@ -0,0 +1,19 @@
+
+// Load in a texture from a sampler2D with an offset and scale
+// See examples/place_texture.gdshader
+vec4 place_texture(sampler2D sampler, vec2 uv, vec2 texture_pixel_size, vec2 offset, vec2 scale) {
+	vec2 texture_size = vec2(textureSize(sampler, 0));
+	// position of current pixel; sample color c
+	vec2 pos = (uv - offset) / (texture_size*texture_pixel_size) / scale;
+	vec4 c = texture(sampler, pos);
+	// top-left bounds
+	vec2 a = offset;
+	// bottom-right bounds
+	vec2 b = offset + (texture_size*texture_pixel_size) * scale;
+	// check bounds
+	if (
+		a.x  < uv.x && a.y  < uv.y
+		&& b.x > uv.x && b.y > uv.y
+	) { return c; } // within bounds -> return color
+	return vec4(0); // not within bounds -> return transparency
+}