#5629 Staff_symbol clean-up

[Dan Eble]

1: Reduce unnecessary access to line-positions property

In some places, Staff_symbol had already checked line-positions before calling line_count (), which checked it again.

2: Remove unused Staff_symbol::line_count () and Staff_symbol_referencer::line_count ()

3: Use ly_scm2floatvector () in Staff_symbol::line_positions ()

4: Make height of a no-line staff [0, 0] rather than empty

This fixes a bug in the output of input/regression/rest-positioning.ly, in which a staff-group bracket failed to extend to a no-line staff.

https://codereview.appspot.com/571210043

[Dan Eble]

Here's the regtest difference. Previously, the bracket did not extend to the first staff.

[Anonymous]

Passes make, make check and a full make doc.

My reg test diffs attached

[Anonymous]

Patch on countdown for Dec 11th

[Anonymous]

Patch counted down - please push.

[Dan Eble]

commit 5cf79d355654fe2fae3720eba84877e13b96abc6
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Fri Dec 6 10:59:26 2019 -0500

    Issue 5629/4: Make height of a no-line staff [0, 0] rather than empty

    This fixes a bug in the output of input/regression/rest-positioning.ly,
    in which a staff-group bracket failed to extend to a no-line staff.

commit 6613cee7a07ba7cc5b975e4d07257a8f090c328d
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Fri Dec 6 10:56:46 2019 -0500

    Issue 5629/3: Use ly_scm2floatvector in Staff_symbol::line_positions

commit 68d339a89f386747e67a5fca92189f6f0f61095e
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Fri Dec 6 10:50:48 2019 -0500

    Issue 5629/2: Remove unused Staff_symbol::line_count ()

    ... and Staff_symbol_referencer::line_count ()

commit c102b83a8f165edf4955dd8e7bd758f639cb54be
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Fri Dec 6 10:45:53 2019 -0500

    Issue 5629/1: Reduce unnecessary access of line-positions property

    In some places, Staff_symbol had already checked line-positions before
    calling line_count (), which checked it again.