Color patch face based upon first vertex (bug #40247). * gl-render.cc (opengl_renderer::patch_tesselator::vertex): Only set per-vertex color if "interp" is set. Use GL_DIFFUSE when setting diffuse lighting (was inccorrectly GL_AMBIENT). * gl-render.cc (draw_patch): If using "flat" shading and no "facecolor" specified then choose the color from the first vertex.

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