Support GL_POLYGON

This can be trivially supported as a triangle fan, since polygons are
guaranteed to be convex and the vertices are specified in the same
order.
The difference will only be noticeable if the rendering mode is set to
GL_LINE, in which case we'll be producing more lines than needed: a
correct implementation would draw just the perimeter, while we'll draw
some internal lines too.

src/gc_gl.c

@@ -2051,6 +2051,7 @@ static unsigned char draw_mode(GLenum mode)
         gxmode = GX_TRIANGLESTRIP;
         break;
     case GL_TRIANGLE_FAN:
+    case GL_POLYGON:
         gxmode = GX_TRIANGLEFAN;
         break;
     case GL_TRIANGLES:
@@ -2059,8 +2060,6 @@ static unsigned char draw_mode(GLenum mode)
     case GL_QUADS:
         gxmode = GX_QUADS;
         break;
-
-    case GL_POLYGON:
     default:
         return 0xff; // FIXME: Emulate these modes
     }