Function to "safely" transform to webmercator. This function is most useful for rendering custom geometries using the CartoDB tiler. Often, transforming a projection like WGS84 can cause issues with extents beyond what are actually valid in webmercator, this attempts to fix those issues.
#### Using the function
Using a box that is nearly the full globe,
```sql
ST_SetSRID(
ST_Envelope(
ST_Collect(
ST_MakePoint(-180,60),
ST_MakePoint(180,-60)
)
),
4326
)
```
We can then convert it to a renderable webmercator geometry.
```sql
SELECT CDB_TransformToWebmercator(
ST_SetSRID(
ST_Envelope(
ST_Collect(
ST_MakePoint(-10,60),
ST_MakePoint(300,-60)
)
),
4326
)
)
```
Would give you back a single valid rectangle in webmercator. Since a longitude of 300 would convert to an unallowed webmercator coordinate, it gets clipped first. Valid extent is WGS84 (-180, -89, 180, 89)
