message: make it easier to zero-out the transferfcn widget values
  The box shown for the widget is extended to essentially go "below zero",
  so you can drag lines to be definitely 0, easily and then the value
  is just clamped to 0.
diff: @@ -163,8 +163,8 @@ void TransferFunction::drawUi()
  ImDrawList *draw_list = ImGui::GetWindowDrawList();
  draw_list->AddRect(canvasPos, canvasPos + canvasSize, ImColor(255, 255, 255));
  - const vec2f view_scale(canvasSize.x, -canvasSize.y);
  - const vec2f viewOffset(canvasPos.x, canvasPos.y + canvasSize.y);
  + const vec2f view_scale(canvasSize.x, -canvasSize.y + 10);
  + const vec2f viewOffset(canvasPos.x, canvasPos.y + canvasSize.y - 10);
  ImGui::InvisibleButton("canvas", canvasSize);
  if (ImGui::IsItemHovered()){
  @@ -257,7 +257,7 @@ void TransferFunction::render()
  sampleColor[j] = clamp(lerp(lit[j]->y - 0.001, (lit[j] + 1)->y - 0.001, t));
  }
  for (size_t j = 0; j < 3; ++j) {
  - palette[i * 4 + j] = sampleColor[j] * 255.0;
  + palette[i * 4 + j] = clamp(sampleColor[j] * 255.0, 0.0, 255.0);
  ospColors[i][j] = sampleColor[j];
  }
  ospAlpha[i].x = x;
  