[Bug report] Descender impacts padding on text layers
### Bug report
#### Expected behavior:

When padding is dependent on text layers on a vertical axis (such as a symbol with only text layer and a rectangle with "[0 x]"), the height of the symbol should stay flexible depending on the line-height of the text layer, but independent of the actual content of the text override.

#### Actual behavior:

The height of the symbol changes depending on if the override text contains letters that have descenders. Only if a descender is present in the override, the height is calculated correctly (irregardless if the original placeholder contains a descender or not). This is also affecting multiple lines. If the last line **doesn't** have a descender it's wrong again.

#### Steps to reproduce:

<img width="719" alt="screenshot 2018-08-01 15 14 14" src="https://user-images.githubusercontent.com/6000697/43523712-9531d758-959d-11e8-86ee-8bf871f389a6.png">


[descender-paddy-bug.sketch.zip](https://github.com/DWilliames/paddy-sketch-plugin/files/2249559/descender-paddy-bug.sketch.zip)



