Fix case of buffer maps in idle writing state - don't generate chunks

* If we generate an unmap chunk on a buffer while idle, it contains
  'dangerous' data set at that point - it means that if we fetch the buffer
  initial contents at frame start, these are actually saved *before* the
  unmap chunk, in the initialisation/creation chunk which contains the
  shadow copy. Then the unmap overwrites with older data.
* When mapping buffers, we should always either update the initial chunk or
  mark the buffer dirty so the initial chunk is refreshed on capture.
* This was changed in commit 99c6719928 and
  was probably ok before then (although going into the serialise function
  was unnecessary and did some redundant copies into a chunk we were going
  to throw away). I'm not sure why I made that change, if it was to fix a
  bug I can't understand how. I guess I'll see if anything breaks.

renderdoc/driver/gl/wrappers/gl_buffer_funcs.cpp

@@ -1650,9 +1650,11 @@ GLboolean WrappedOpenGL::glUnmapNamedBufferEXT(GLuint buffer)
 				}
 				else
 				{
-					SCOPED_SERIALISE_CONTEXT(UNMAP);
-					Serialise_glUnmapNamedBufferEXT(buffer);
-					record->AddChunk(scope.Get());
+					// if we are here for WRITING_IDLE, the app wrote directly into our backing
+					// store memory. Just need to copy the data across to GL, no other work needed
+					void *ptr = m_Real.glMapNamedBufferRangeEXT(buffer, (GLintptr)record->Map.offset, GLsizeiptr(record->Map.length), GL_MAP_WRITE_BIT);
+					memcpy(ptr, record->Map.ptr, record->Map.length);
+					m_Real.glUnmapNamedBufferEXT(buffer);
 				}
 				
 				break;