Reworked LLPostProcess and implemented FBO support (much faster if multiple post shaders are enabled, or need a lot of passes).

Tweaked LLRenderTarget to support depth textures if FBO support is lacking.
Prefer LLRenderTarget::getFBO() over LLRenderTarget::sUseFBO when determining how to handle a specific LLRenderTarget object. (Decoupling to simplify logic without having to track a global)

indra/newview/app_settings/windlight/postprocesseffects.xml

@@ -174,6 +174,8 @@
 			<boolean>0</boolean>
 			<key>enable_gauss_blur</key>
 			<boolean>0</boolean>
+      <key>enable_posterize</key>
+      <boolean>0</boolean>
 			<key>gauss_blur_passes</key>
 			<integer>2</integer>
 			<key>extract_high</key>
@@ -186,6 +188,8 @@
 			<real>0.40000000000000002</real>
 			<key>saturation</key>
 			<real>1</real>
-		</map>
+      <key>posterize_layers</key>
+      <real>10</real>
+    </map>
 	</map>
 </llsd>