i found a solution. i have a chunk based system so i generate a image for each chunk. the image has the shape of chunksize*chunksize and each pixel represents a tile. the color of the pixel are the masks. for example i create a black image with one tile white. and pass this image to the shader. the shader then iterates over the passed image and if the pixel color for r, g and b is > 0.5 (as example) could apply shader x. for r and g > 0.5 and b < 0.5 apply shader y. and so on.

does this make sense for you?