drm_hwcomposer: Add GetExtraClientRange method

Separate the correction of client range into the separate method.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/backend/Backend.h b/backend/Backend.h
index 8658efa..a32c57f 100644
--- a/backend/Backend.h
+++ b/backend/Backend.h
@@ -36,6 +36,10 @@
  protected:
   uint32_t CalcPixOps(const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,
                       size_t first_z, size_t size);
+  std::tuple<int, int> GetExtraClientRange(
+      DrmHwcTwo::HwcDisplay *display,
+      const std::vector<DrmHwcTwo::HwcLayer *> &layers, int client_start,
+      size_t client_size);
 };
 }  // namespace android