Added defines: GL_ALPHA4/8/12/16 in osg/Image, this will fix compilation error while building for GLES2
Added defines: GL_ALPHA4/8/12/16 in osg/Image, this will fix compilation error while building for GLES2

openscenegraph/osg,scrawl/osg,scrawl/osg,openscenegraph/osg,openscenegraph/osg,scrawl/osg,scrawl/osg,openscenegraph/osg