evas: Drop unused buffer from textblock filters

evas: Drop unused buffer from textblock filters

This is a first step before implementing some form of caching of
those output buffers. At the moment, it very aggressively deletes
any buffer that falls outside the clip of the textblock object.
Note that this is better in terms of memory usage but way worse
in terms of render performance (eg. scrolling). If a textblock
is a proxy source then we keep all the buffers (the entire object
is to be rendered).

+ fix a crash