Just reading over this and I am curius, does this shader layer images together?
If so, does that mean it would be possible to do this with a shader and if so how fast would it be compared to an extension?

image 1
image 2, insert a greyscale image into the alpha channel of image
layer image 2 over image 1
image 3, insert a greyscale image into the alpha channel of image
layer image 3 over image 2

I am thinking of an example for tile based terrain blending.

Say I have 4 tiles, grass, water, dirt and sand and I have 15 alphas for each possible edge transition.
What I am hoping is that this would allow me to create different blends using the above method.

Is this possible??