SDL: Add new pixel perfect stretch mode for OpenGL

This mode ensure that an integral scaling is used both for the width
and for the height. Compared to the old pixel perfect stretch mode
it thus differs in the way it handles the aspect ratio correction.
It may stretch or squeeze the vertical direction to snap it to a
multiple of the original game height.

backends/graphics/opengl/opengl-graphics.cpp

@@ -241,6 +241,7 @@ namespace {
 const OSystem::GraphicsMode glStretchModes[] = {
 	{"center", _s("Center"), STRETCH_CENTER},
 	{"pixel-perfect", _s("Pixel-perfect scaling"), STRETCH_INTEGRAL},
+	{"even-pixels", _s("Even pixels scaling"), STRETCH_INTEGRAL_AR},
 	{"fit", _s("Fit to window"), STRETCH_FIT},
 	{"stretch", _s("Stretch to window"), STRETCH_STRETCH},
 	{"fit_force_aspect", _s("Fit to window (4:3)"), STRETCH_FIT_FORCE_ASPECT},