Simplify parsing of affine matrix arguments

[cmb69]

This code repetition is prone to errors and makes the code harder to read than necessary. We simplify at the cost of making parsing of ints slightly less performant (what should not really matter in practice).

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So far this is fully backwards compatible. However, given that the $clip argument of imageaffine() uses zend_get_long() without further type checks, we could use zend_get_double() for the matrix arguments also without further type checks. Or perhaps being more strict. As is, the documentation (which is highly incomplete for now) would need to go to great lengths to explain which types are supported for the array elements and how they are handled.

[Girgias]

MSTM, should we add a zval_try_get_double API?

[cmb69]

should we add a zval_try_get_double API?

Sounds like a good idea. It might be also reasonable to somehow merge the implementations of zval_get_long() and zval_try_get_long() etc.; there's quite some code duplication there.