Clarify role of vertex data in examples

This adds a simple comment that clarifies how the data in the
vertex_format struct relates to the shader.
I struggled a bit before I understood this, so maybe it will be helpful
for others to have that piece of information in the examples (at least
until more comprehensive documentation exists).
Add prim_type argument to Mesh::from
Add prim_type argument to create_mesh

I think rendering different primitive types is common enough to justify
adding this argument to create_mesh. This has the added benefit of
clearly showing users that learn from the examples how the primitive
type can be set.
Merge pull request #251 from hannobraun/vertex-data-comment

Clarify role of vertex data in examples
Merge pull request #250 from hannobraun/primitive-arg

Expose primitive type through create_mesh
Improved buffer updating interface
Merge pull request #253 from kvark/buffer

Improved buffer updating interface
Made `gfx_macros`'s macros no longer depend on a `gfx` module in the top
level.

This is achieved by inserting `extern crate gfx;` imports at appropiate
places during expansion.

These imports use unique generated identifiers, and thus don't
conflict with each other or user code.

This also changes gfx-test to not having a `gfx` module in scope, thus
ensuring no new breakage sneaking in.
Merge pull request #255 from Kimundi/fix-shaders

Made `gfx_macros`'s macros no longer depend on a `gfx` module in the top
Make fields of `device::Capabilities` public

Since we only expose the capabilities behind a `&`, consumers of the API can't
mutate the capabilities accidentally. This also allows consumers to do their
own querying.
device: gl: tex: clean up some absolute references with imports
device: clean up warnings
render: add a comment to unused state
Merge pull request #257 from cmr/various-improvements

Some fixes
Fix Shader version to shader model mapping
Merge pull request #258 from Kimundi/master

Fix Shader version to shader model mapping
add terrain demo
updated Makefile to build terrain demo
Merge pull request #259 from csherratt/master

add terrain demo