Keep your MTLBuffer struct buffer size aligned with this easy to use function

alignbuffer.swift

    /*
     * `bufferSize` is the total size of your buffer when adding up the sizes of your struct's properties (not alignments)
     * `alignment` is the largest alignment for your struct. That is one of your struct's properties will have the largest
     * alignment value out of all the alignment values for each of your structs properties. Use that value for the second 
     * argument.
     * 
     * To see the alignment of types see here: https://developer.apple.com/library/ios/documentation/Metal/Reference/MetalShadingLanguageGuide/data-types/data-types.html#//apple_ref/doc/uid/TP40014364-CH2-SW15
     *
     * Example:
     * 
     * In swift you may have the following struct:
     *
     * struct RenderData {
     *    var resolution: [Float32]
     *    var cameraMatrix: [Float32]
     * }
     *
     * In metal it will look like this:
     *
     * struct RenderData {
     *  packed_float2 resolution;
     *  float4x4 cameraMatrix;
     * }
     *
     * `resolution` is a packed_float2 and has a size of 8 bytes and an alignment of 4 bytes.
     * `cameraMatrix` is a float4x4 matrix with a size of 64 bytes and an alignment of 16 bytes.
     *
     * The largest alignment in this struct is 16. The total size of the MTLBuffer must be a multiple of 16.
     * The total struct size will be 64 + 8 = 72, but 72 % 16 != 0 and this will blow up.
     *
     * let bufferSize = 64 + 8;
     * let matrixAlignment = 16;
     * let totalBufferSize = alignBufferSize(bufferSize, matrixAlignment);
     *
     * `totalBufferSize` will be 80.
     *
     * Now use this value to make your MTLBuffer:
     *
     *  let buffer = device.makeBuffer(length: totalBufferSize, options: [])
     *
     */
    func alignBufferSize(bufferSize: Int, alignment: Int) -> Int {
        let alignmentError = bufferSize % alignment;
        if (alignmentError == 0) {
            return bufferSize
        }
        return bufferSize + (alignment - alignmentError)
    }