wined3d: Don't use the GL texture flags to determine if a format is a depth stencil in CheckDepthStencilCapability().

The OpenGL implementation might not support ARB_depth_texture, but that
doesn't mean we can't use the format for the depth buffer.

"surface" might point to the frontbuffer while we're rendering to the
backbuffer, etc.

Fixes building on Snow Leopard.

Marlett uses FW_MEDIUM (500) and current code allows only FW_REGULAR and
FW_BOLD. New heuristics detects if flags and usWeightClass in the font match.

Freetype has a similar consistency check.