Fix ligatures

[lpugin]

Following problems in the ligatures from the test-suite have been reported by @karend27:

• [ ] ligature-006.mei, the first and fourth examples in black mensural: I actually think both of these should be rendered the way that you rendered the white mensural versions. Currently having the two notes stacked and sharing one descending stem it is unclear which note has the stem. The same is true of the fifth example in ligature-045.mei.
• [ ] ligature-026.mei: How would the penultimate note of this ligature be rendered if you had not specified @lig="obliqua" - seems like it's going to have to be oblique if the last note is oblique (due to its being a breve). So does the penultimate note need to be specified in the attribute? (same is true of the penultimate note in ligature-043.mei) [In ligature-033.mei the penultimate note does not have an "obliqua" attribute]
• [x] ligature-033.mei: I don't think I understand why @lig = "obliqua" is specified on the first note here?
• [x] ligature-040.mei: wouldn't the default rendering for the first two notes of this ligature be two square notes, and if you wanted to render them oblique you would need to specify @lig = "obliqua" on the first note? And the same with ligature-024.mei, and ligature-039.mei and ligature-044.mei
• [ ] ligature-045.mei, third example: logically I guess this is correct, but it looks pretty unusual to me: more usually you wouldn't have the first two notes oblique for ascending - the first two notes would be square notes (but I guess this one is fine, because I understand this is not the default rendering here, and is specifically being forced through the specification of @lig="obliqua").