Limit exports from OpenGL backend
bump upper limit on containers
Add several examples / test cases, mostly testing examples added in
later commits

These examples come from the manual:
- fillRule
- transparency
- line styling

In addition:
- Add unfilled example to fillRule.hs
- add path-superposition example to fillRule.hs
- Expand lines test case to test path closing
Handle opacity / transparency

- Apply Opacity Attribute when setting fillColor
- set display modes
- set GL blending in defaultMain
Parameterize GlPrim on P2

This brings GlPrim closer to Diagrams, and farther from the underlying OpenGL.
It also means the type system can keep track of the dimension, and therefore
makes the type easier to understand.
Merge Andrey's GLU tesselation code, using the State Monad.

The State Monad is used here to get around the way withStyle is
called.  We'd need to have the fillRule when constructing GlPrims from
Paths.  In particular, several Trails which are part of one Path are
tesselated differently from seperate Paths.

Andrey notes that the GLU tesselation routines have several
shortcomings.  It would still be nice to replace this with a native
implementation at some point.
Add support for dashed lines, LineCap, LineJoin Attributes

Convert lines to polygons, so this geometry can be calculated directly.
Allow stylePrim to return more GlPrims than it received.

Code by Andrey Yankin
Comments and cleanup by Daniel Bergey