[HdSt] Reorganized resource binding for indirect draw batches

Implemented common methods for binding and unbinding draw item
resources used for drawing and also for instanced and non-instanced
view frustum culling.

The two methods previously used for instanced and non-instanced
view frustum culling have been combined, since most of the code
is common to both methods. Also, moved some culling specific code
from from the Prepare() method down into the culling implementation.

(Internal change: 2205708)

pxr/imaging/hdSt/drawBatch.cpp

@@ -300,6 +300,12 @@ HdSt_DrawBatch::_GetDrawingProgram(HdStRenderPassStateSharedPtr const &state,
     return _program;
 }
 
+bool
+HdSt_DrawBatch::_DrawingProgram::IsValid() const
+{
+    return _glslProgram && _glslProgram->Validate();
+}
+
 bool
 HdSt_DrawBatch::_DrawingProgram::CompileShader(
         HdStDrawItem const *drawItem,

pxr/imaging/hdSt/drawBatch.h

@@ -123,6 +123,9 @@ class HdSt_DrawBatch
     public:
         _DrawingProgram() {}
 
+        HDST_API
+        bool IsValid() const;
+
         HDST_API
         bool CompileShader(
                 HdStDrawItem const *drawItem,