Consistently use the padded element size

[javagl]

According to 3.6.2.4. Data Alignment of the specification, padding bytes have to be inserted at the end of columns of certain MATn types. The general difference between an "element size" and an "element size with padding" may be that

• the element size for a MAT2 with GL_BYTE is 4
• the element size with padding for a MAT2 with GL_BYTE is 8

This was already anticipated, e.g. with the ElementType class. But some classes (most importantly the BufferStructureBuilder) did not anticipate this, and did write 'wrong' data into the buffers when the inputs have been one of the affected matrix types.