Proto commits in gwaldron/osgearth

These 12 commits are when the Protocol Buffers files have changed:

Commit:4db9486
Author:Jason Beverage

Work on MapboxGLImageLayer. Support for MapboxGL fonts, metatiling and bug fix for != filters.

The documentation is generated from this commit.

Commit:5aafd24
Author:gwaldron

Update/fix issues with TileSource_Layer compability layers

Commit:a65f61d
Author:gwaldron
Committer:gwaldron

Namespace refactor, part 2

Commit:43628e3
Author:gwaldron

Namespace refactor, part 2

Commit:89f06d6
Author:gwaldron

Upgrade shaders to single-file and remove the old disabled drivers folder

Commit:d81dd97
Author:Jason Beverage

Removed old feature_mapnikvectortiles from osgEarthDriversDisabled.

Commit:d235ad4
Author:Jason Beverage

Added compile time -DOSGEARTH_HAVE_MVT switch.

Commit:8772d76
Author:Jason Beverage

move mvt protobuf code

Commit:8dee4ec
Author:Jason Beverage

Added disabled GeoBuff driver.

Commit:078c9a1
Author:Jason Beverage

Added the mapnikvectortiles driver back to the list of enabled drivers.

Commit:2a434b6
Author:Jason Beverage

Disabled the feature_mapnikvectortiles driver for the 2.7 release. It's got a few bugs and not really complete yet.

Commit:0d6d448
Author:Jason Beverage

Using CMake to run the protoc command at buildtime to generate the protobuf files instead of including them in the repo.