This is no longer used. It was previously used in two places to perform
an open-coded wined3d_context_gl_draw_textured_quad(), but the latter checks
wined3d_texture_gl.target instead, which is set to GL_TEXTURE_2D or
GL_TEXTURE_RECTANGLE_ARB under the exact same conditions, making this flag
redundant even at the time.

The two uses were removed in 012f9b03 [which changed the relevant code to use
wined3d_context_gl_draw_textured_quad() instead], and cdacbd98 [which removed
the relevant code entirely.]