mesa: Recompute LegalTypesMask if the GL API has changed

The current code computes ctx->Array.LegalTypesMask just once,
however, computing this needs to consider ctx->API so we need
to make sure that the API for that context has not changed if
we intend to reuse the result.

The context API can change, at least, if we go through
_mesa_meta_begin, since that will always force
API_OPENGL_COMPAT until we call _mesa_meta_end. If any
operation in between these two calls triggers a call to
update_array_format, then we might be caching a value for
LegalTypesMask that will not be right once we have called
_mesa_meta_end and restored the context API.

Fixes the following 18 dEQP tests in i965:
dEQP-GLES3.functional.vertex_arrays.single_attribute.strides.fixed.*