Fixes the buffer generated for mitred joins.

Previously the buffer generated did not correctly maintain the buffer distance with mitred joins (see libgeos/geos#531).

Example 1

LINESTRING (20 50, 20 80, 50 80)

• Buffer distance = 10, Flat Cap, Mitre Limit = 2
  
• Buffer distance = 10, Flat Cap, Mitre Limit = 1
  
• Buffer distance = 10, Flat Cap, Mitre Limit = 0
  

Example 2

LINESTRING (10 80, 70 50, 10 50)

• Buffer distance = 10, Flat Cap, Mitre Limit = 5
  
• Buffer distance = 10, Flat Cap, Mitre Limit = 5
  

Signed-off-by: Martin Davis [email protected]