Fix struct variable size with out-of-order offsets

[spencer-lunarg]

This is follow-up for #263

I realize the struct size itself is still wrong

name     : ubo (UniformBufferObject)
    // size = 16, padded size = 16
    struct UniformBufferObject {
        float a; // abs offset = 0, rel offset = 0, size = 4, padded size =  4 UNUSED
        float b; // abs offset = 4, rel offset = 4, size = 4, padded size = 12 UNUSED
    } ubo;      

name     : ubo2 (UniformBufferObject2)
    // size = 4, padded size = 4
    struct UniformBufferObject2 {
        float b; // abs offset = 4, rel offset = 4, size = 4, padded size = 12 UNUSED
        float a; // abs offset = 0, rel offset = 0, size = 4, padded size =  4 UNUSED
    } ubo2;     

The size of ubo2 is not // size = 4, padded size = 4 and should be 16

[spencer-lunarg]

first commit is just removing this line, ParseDescriptorBlockVariableSizes starts with a check for this