Fix variable name that changed in recent PR in master. (#749)

Author: phi-lira

com.unity.render-pipelines.universal/Shaders/PostProcessing/BokehDepthOfField.shader

@@ -38,10 +38,10 @@ Shader "Hidden/Universal Render Pipeline/BokehDepthOfField"
             uint h;
 #if defined(SHADER_API_GLCORE)
             // GetDimensions will use textureQueryLevels in OpenGL and that's not
-            // supported in OpenGL 4.1 or below. In that case we use _MainTex_TexelSize
+            // supported in OpenGL 4.1 or below. In that case we use _SourceTex_TexelSize
             // which is fine as we don't support dynamic scaling in OpenGL.
-            w = _MainTex_TexelSize.z;
-            h = _MainTex_TexelSize.w;
+            w = _SourceTex_TexelSize.z;
+            h = _SourceTex_TexelSize.w;
 #elif defined(UNITY_STEREO_INSTANCING_ENABLED) || defined(UNITY_STEREO_MULTIVIEW_ENABLED)
             uint x;
             _CameraDepthTexture.GetDimensions(w, h, x);