Non square matrices related fixes

Non square matrices related fixes

- Implemented proper VariableRegisterCount (we were using row
  count instead of column count to get the number of registers)
  and VariableRegisterSize.
- Matrix to matrix copies now clear the correct rows of the
  destination matrix when needed
- Added registerSize helper function to type to help clarify
  this for matrices.
- Added missing member initializations in TType constructor

Change-Id: Ic880815515c7d12ad12e44f1392aa6892caa953f
Reviewed-on: https://swiftshader-review.googlesource.com/3718
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>