fix angle calculation

Fixes following bug I found while playing with text rendering.

Angle of the last character is not always correctly calculated. Here is example of [text-halign-800-800-2.0-agg-reference.png](https://raw.githubusercontent.com/mapnik/mapnik/5688a938589db2d17e398a12cd7b25e6ce9d48c3/tests/visual_tests/images/text-halign-800-800-2.0-agg-reference.png) reference image:
![00](https://cloud.githubusercontent.com/assets/1950911/4061952/382a4250-2dfc-11e4-8ca8-dc6807badcc5.png)
After this fix:
![00_](https://cloud.githubusercontent.com/assets/1950911/4061953/38466110-2dfc-11e4-86cc-9b9b9717041c.png)

[lines-4-200-200-2.0-agg-reference.png](https://github.com/mapnik/mapnik/blob/5688a938589db2d17e398a12cd7b25e6ce9d48c3/tests/visual_tests/images/lines-4-200-200-2.0-agg-reference.png):
![lines-4-200-200-2 0-agg-reference](https://cloud.githubusercontent.com/assets/1950911/4062000/9e47131a-2dfc-11e4-88e4-f5361f19fd9f.png)
After:
![lines-4-200-200-2 0-agg](https://cloud.githubusercontent.com/assets/1950911/4062005/b32863e2-2dfc-11e4-91f8-94d98246376a.png)

[lines-3-800-800-1.0-agg-reference.png](https://github.com/mapnik/mapnik/blob/5688a938589db2d17e398a12cd7b25e6ce9d48c3/tests/visual_tests/images/lines-3-800-800-1.0-agg-reference.png):
![01](https://cloud.githubusercontent.com/assets/1950911/4062091/4edff19c-2dfd-11e4-8464-5ef87719c552.png)
After:
![01_](https://cloud.githubusercontent.com/assets/1950911/4062092/4ef87816-2dfd-11e4-8ba9-d7edd89ca35a.png)

[lines-5-800-800-1.0-agg-reference.png](https://github.com/mapnik/mapnik/blob/5688a938589db2d17e398a12cd7b25e6ce9d48c3/tests/visual_tests/images/lines-5-800-800-1.0-agg-reference.png):
![02](https://cloud.githubusercontent.com/assets/1950911/4062228/51e3612a-2dfe-11e4-82bf-e15d2987f6bc.png)
After:
![02_](https://cloud.githubusercontent.com/assets/1950911/4062229/51f9a746-2dfe-11e4-97d7-465ded2d7652.png)

New placement appeared in [collision-600-400-2.0-agg-reference.png](https://github.com/mapnik/mapnik/blob/5688a938589db2d17e398a12cd7b25e6ce9d48c3/tests/visual_tests/images/collision-600-400-2.0-agg-reference.png):
![03](https://cloud.githubusercontent.com/assets/1950911/4062367/4bf7e370-2dff-11e4-80cc-f1b4d4475f83.png)
After:
![03_](https://cloud.githubusercontent.com/assets/1950911/4062368/4bf87736-2dff-11e4-9f95-eac3881c2bf9.png)

It does not change any unrelated visual tests.