Generate shadows before lighting in deferred rending

[gongminmin]

Refactor deferred rendering framework. Move the shadow generation (including RSM generation) in between G-Buffers and lighting. This will limit the number of shadowing light into 4, texture projection light into 1.

[gongminmin]

New algorithm implemented by Yuan Li in changeset 3040d282ad20da601c21dbe365c69fada6386371.

for each shadowed light:

generate shadow map or reflective shadow map.
convert sm to linear.
blur linear sm.

for each view port:

generate g-buffer.
shadowing from sm.
lighting.