Previously, to determine if a texture format was renderable, the
texturable and renderable fields had to be and-ed together.  This caused
issues for formats such as D24S8 which can be renderable but not
texturable depending on available extensions.

Made the renderable flag a complete check that may be different than the
textureable flag and removed assumptions that a format is texturable if
renderable from the code.

GL_DEPTH24_STENCIL8 now checks for ANGLE_depth_textures for texturability
and ANGLE_depth_textures or OES_packed_depth_stencil for renderability.

BUG=angle:752

Change-Id: I6d197cee72cc249e5996fa395303bdf43d246a87
Reviewed-on: https://chromium-review.googlesource.com/219093Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>