Add error message for assertion in ProgramHandle::fill_params
Merge pull request #206 from tomaka/error-msg-fill-params

Add error message for assertion in ProgramHandle::fill_params
Fix gl_init events
Merge pull request #207 from tomaka/fix-gl-init-events

Fix gl_init events
Implemented a DrawList trait, its GL backend, added a simple example that will show it off.
First steps at render frontend implementation, bits of framebuffer support.
Cleaned up large chunks of renderer, including the Dispatcher. Implemented mesh binding for the front-end.
Implemented resource deletion, program linking and connection, mesh creation.
Lots of renaming of 'dev' to 'back' to avoid 'device::dev' pattern. Split ProgramMeta into ProgramInfo and the internal name, which are combined into ProgramHandle.
Implemented proper handles for textures, samplers, and surfaces. Deleted all the device and renderer related cruft.
Made buffers and shaders to use safe handlers. Fixed some doc annotations.
Fixed the tests
Removed draw_list example. Fixed triangle to work.
Renamed FrontEnd to DrawList, mostly fixed the cube example.
Fixed tabs. Implemented BackEndHelper, link_program, update_texture.