Get a block of pixel colors.
mipWidth=max(1,width>>miplevel)
and similarly for height.Using GetPixels can be faster than calling GetPixel repeatedly, especially
for large textures. In addition, GetPixels can access individual mipmap levels.Get a block of pixel colors.
GetPixels above; it does not return the whole
mip level but only blockWidth by blockHeight region starting at x,y.
The block must fit into the used mip level. The returned array is blockWidth*blockHeight size.