De-duplicate shader code for encoding and decoding normals to/from gbuffer format.

indra/llrender/llglslshader.cpp

indra/llrender/llglslshader.h

indra/llrender/llshadermgr.cpp

.../newview/app_settings/shaders/class1/deferred/alphaF.glsl

...newview/app_settings/shaders/class1/deferred/avatarF.glsl

...view/app_settings/shaders/class1/deferred/blurLightF.glsl

...a/newview/app_settings/shaders/class1/deferred/bumpF.glsl

...p_settings/shaders/class1/deferred/diffuseAlphaMaskF.glsl

...ngs/shaders/class1/deferred/diffuseAlphaMaskIndexedF.glsl

...ngs/shaders/class1/deferred/diffuseAlphaMaskNoColorF.glsl

...ewview/app_settings/shaders/class1/deferred/diffuseF.glsl

...app_settings/shaders/class1/deferred/diffuseIndexedF.glsl

...wview/app_settings/shaders/class1/deferred/materialF.glsl

...pp_settings/shaders/class1/deferred/multiPointLightF.glsl

...app_settings/shaders/class1/deferred/multiSpotLightF.glsl

...iew/app_settings/shaders/class1/deferred/pointLightF.glsl

...ew/app_settings/shaders/class1/deferred/softenLightF.glsl

...view/app_settings/shaders/class1/deferred/spotLightF.glsl

...w/app_settings/shaders/class1/deferred/sunLightSSAOF.glsl

...ewview/app_settings/shaders/class1/deferred/terrainF.glsl