perspective matrix WIP
feature selection: call callback even when no feature found
feature selection: set w coord to 1 when short-circuiting selection buffer rendering
feature selection: fix vector type for no-op selection buffer render
perspective matrix: improve depth near/far clip plane range, use gl-matrix for building perspective matrix, replace meter_view matrix
Merge remote-tracking branch 'origin/traceur' into camera-es6

Conflicts:
	dist/tangram-worker.min.js
	dist/tangram.debug.js
	dist/tangram.min.js
add a metersPerPixel method, will transition constants to it (better for fractional zoom)
GLProgram: add support for global shader transforms (like global #defines)
GLProgram: helper function to add one or more global shader transforms at once
add method to clear global shader transforms for a given key
remove old camera UI
move camera projections to new module, add intial config to scene definition
add camera config back into demo UI
use ES6 spread operator for GL uniform setter
Merge branch 'traceur' into camera-es6
Merge branch 'master' into camera-es6

Conflicts:
	dist/tangram-worker.debug.js
	dist/tangram-worker.min.js
	dist/tangram.debug.js
	dist/tangram.min.js
update ES6 module import style
update build
Merge branch 'master' into camera-es6
allow focal length to be configured in scene defition, and support interpolated lengths between zoom ranges