i965: Set a proper _BaseFormat for window system renderbuffers in ES.

i965: Set a proper _BaseFormat for window system renderbuffers in ES.

intel_alloc_private_renderbuffer_storage did:

   rb->_BaseFormat = _mesa_base_fbo_format(ctx, internalFormat);

Unfortunately, internalFormat was usually an unsized format (such as
GL_DEPTH_COMPONENT).  In OpenGL ES, _mesa_base_fbo_format() refuses to
accept unsized formats, and returns 0 rather than a real base format.

This meant that we ended up with a completely bogus rb->_BaseFormat for
window system buffers on OpenGL ES.  All other renderbuffer allocation
functions in intel_fbo.c instead use the mesa_format, and do:

   rb->_BaseFormat = _mesa_get_format_base_format(...);

We can do likewise, using rb->Format.  This appears to work just fine.

dEQP-GLES3.functional.state_query.fbo.framebuffer_attachment_x_size_initial
failed, as it tried to perform a GL_FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE query
on the window system depth buffer.  That query relies on a proper
rb->_BaseFormat being set, so it broke because rb->_BaseFormat was 0 due
to the above bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94458
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>