Have __glImageSize handle format GL_DEPTH_STENCIL_NV the same way as the server does

There is a mismatch between the way the X server and GLX library
calculates the image size for format GL_DEPTH_STENCIL(|_NV|_EXT)

See https://bugs.freedesktop.org/show_bug.cgi?id=30102

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>

src/glx/compsize.c

@@ -71,6 +71,7 @@ __glElementsPerGroup(GLenum format, GLenum type)
    case GL_422_REV_EXT:
    case GL_422_AVERAGE_EXT:
    case GL_422_REV_AVERAGE_EXT:
+   case GL_DEPTH_STENCIL_NV:
    case GL_YCBCR_422_APPLE:
    case GL_LUMINANCE_ALPHA:
       return 2;