mtsgui: ability to view multi-channel EXRs

include/mitsuba/core/bitmap.h

src/libcore/bitmap.cpp

src/libhw/glrenderer.cpp

src/librender/trimesh.cpp

src/mtsgui/common.h

src/mtsgui/glwidget.cpp

src/mtsgui/mainwindow.cpp

src/mtsgui/sceneloader.cpp