mesa: Dynamically allocate the matrix stack.

By allocating and initializing the matrices at context creation, the OS
couldn't even overcommit the pages.  This saves about 63k (out of 946k) of
maximum memory size according to massif on simulated vc4
glsl-algebraic-add-add-1.  It also means we could potentially relax the
maximum stack sizes, but that should be a separate commit.

v2: Drop redundant Top update, explain why the stack is small at init
    time.

Reviewed-by: Brian Paul <[email protected]>

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