Fix spawnBounds to be centred on co-ordinate
Added global gammaCorrection option to pc.scene.Scene, Removed gammaCorrection from particle system component. WIP
Added gamma option to phong materials
Added updateShader function to particle material so that it is updated when the scene's gammaCorrection flag changes
Merge from master
[VERSION] v0.159.0-dev
Merge branch 'master' of github.com:playcanvas/engine

Conflicts:
	examples/particle_system/simple/index.html
	src/framework/components/particlesystem/particlesystem_data.js
	src/graphics/programlib/chunks/particleUpdaterStart.ps
Merge pull request #60 from playcanvas/feature/global-gamma-correction

Global gamma correction
[RELEASE] v0.158.6