Fix implicit cast warning
Fix warning with Clang:
[1/3] Building CXX object IO/ExportP...tPDF.dir/vtkPDFContextDevice2D.cxx.o /home/michael/Dev/vtk/vtk/IO/ExportPDF/vtkPDFContextDevice2D.cxx:1822:33: warning: implicit conversion from 'int' to 'float' changes value from 2147483647 to 2147483648 [-Wimplicit-int-float-conversion] if (this->TextureBounds[0] == VTK_INT_MAX || this->TextureBounds[1] == VTK_INT_MIN ||