remove optimisations in PageState#text_rendering_matrix
remove optimisations in PageState#text_rendering_matrix

* by adding an optimisation to PageState#multiply we get fewer object
  allocations in most cases, which means better performance. It also
  means the code becomes more readable
* PageState#multiply! now recognises when a matrix is being multiplied
  by the 'identity matrix', which means no change is required

nmadura/pdf-reader,eitoball/pdf-reader,yob/pdf-reader,boutil/pdf-reader,yob/pdf-reader,gordonbisnor/pdf-reader,tdenovan/pdf-reader,yob/pdf-reader,nifarius/pdf-reader,tardate/pdf-reader