vo_gl: reject MS Windows native OpenGL as software rasterizer

If the graphics driver doesn't provide its own OpenGL implementation,
applications get Microsoft's OpenGL emulation. Even if it should be the
case that it's not strictly a software renderer, it provides OpenGL 1.1
only, no shaders in any form, and has other limitations that make it
almost completely useless for mplayer.

libvo/vo_gl.c

@@ -432,8 +432,10 @@ static int isSoftwareGl(struct vo *vo)
 {
     struct gl_priv *p = vo->priv;
     const char *renderer = p->gl->GetString(GL_RENDERER);
+    const char *vendor = p->gl->GetString(GL_VENDOR);
     return !renderer || strcmp(renderer, "Software Rasterizer") == 0 ||
-           strstr(renderer, "llvmpipe");
+           strstr(renderer, "llvmpipe") ||
+           strcmp(vendor, "Microsoft Corporation") == 0;
 }
 
 static void autodetectGlExtensions(struct vo *vo)