loadPNG(string fname,int depth)

Loads a PNG file into a tensor.

If depth=1 the PNG is converted into a grey-scale image, and the tensor is an mxn where m is the width and n is the height of the image.

If depth=3 the PNG is converted in RGB color, and the tensor is an m x n x 3, where the last dimension indexes the RGB components of the image.

If depth=4 the tensor is an m x n x 4 where the last dimension indexes the RGBA components of the image.

Here is a typical example:

	w=gfx.Window();
	x=gfx.loadPNG('logo.png',3);	
	w:blit(x);