Get a block of pixel colors.
This function returns an array of pixel colors of the whole
mip level of the texture.
The returned array is a flattened 2D array, where pixels are laid out left to right,
top to bottom (i.e. row after row). Array size is width by height of the mip level used.
The default mip level is zero (the base texture) in which case the size is just the size of the texture.
In general case, mip level size is 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.
This function is an extended version of 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.