QtGui/QtOpenGL: Fix compiler warnings by MSVC2015 (64bit).

opengl\qtriangulatingstroker.cpp(130): warning C4305: '=': truncation from 'double' to 'float'
opengl\qtriangulatingstroker.cpp(546): warning C4305: '=': truncation from 'double' to 'float'
opengl\qopenglshaderprogram.cpp(1779): warning C4312: 'reinterpret_cast': conversion from 'int' to 'const void *' of greater size
qglshaderprogram.cpp(1709): warning C4312: 'reinterpret_cast': conversion from 'int' to 'const void *' of greater size

Change-Id: If1de629aada308d18c3641b504d74d3294b1e8bf
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>