COMP: Fix narrowing warnings in qSlicerSimpleMarkupsWidget

This commit addresses the following warnings:

/home/jcfr/Projects/Slicer/Modules/Loadable/Markups/Widgets/qSlicerSimpleMarkupsWidget.cxx:240:42: warning: narrowing conversion of ‘color.QColor::red()’ from ‘int’ to ‘double’ inside { } is ill-formed in C++11 [-Wnarrowing]
double rgbDoubleVector[3] = {color.red(),color.green(),color.blue()};
^
/home/jcfr/Projects/Slicer/Modules/Loadable/Markups/Widgets/qSlicerSimpleMarkupsWidget.cxx:240:56: warning: narrowing conversion of ‘color.QColor::green()’ from ‘int’ to ‘double’ inside { } is ill-formed in C++11 [-Wnarrowing]
double rgbDoubleVector[3] = {color.red(),color.green(),color.blue()};
^
/home/jcfr/Projects/Slicer/Modules/Loadable/Markups/Widgets/qSlicerSimpleMarkupsWidget.cxx:240:69: warning: narrowing conversion of ‘color.QColor::blue()’ from ‘int’ to ‘double’ inside { } is ill-formed in C++11 [-Wnarrowing]
double rgbDoubleVector[3] = {color.red(),color.green(),color.blue()};

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24457 3bd1e089-480b-0410-8dfb-8563597acbee