Add reflectivity map support for Side-Scan Sonar

[vsjays]

Summary

Adds support for a reflectivity map used by the Side-Scan Sonar (SSS)
renderer. Previously the sonar return intensity was driven only by the surface normal
(and material restitution); a look can now supply a texture that modulates the returned
intensity per-texel, enabling spatially varying acoustic reflectivity across a surface.

What changed

New shaders (derived from sonarInputUv.frag):

• sonarInputRefl.frag — reflectivity map only (geometric normal + reflectivity texture).
• sonarInputReflUv.frag — reflectivity map and normal map combined.

Look / material plumbing — a reflectivity_map texture flows through the full chain:

• ScenarioParser parses the new reflectivity_map attribute on <look> elements.
• SimulationManager::CreateLook and OpenGLContent::CreatePhysicalLook gained a
  reflectivityTexturePath / reflectivityMapPath parameter.
• Look struct now holds a reflectivityMap texture handle; new
  TEX_MAT_REFLECTIVITY (Unit 18) texture slot added.

Sonar rendering (OpenGLSSS / OpenGLSonar):

• sonarInputShader_ array expanded from 2 → 4 to cover the
  {plain, normal, reflectivity, normal+reflectivity} shader combinations.

• Per object, the shader is now selected based on whether the look has a normal map
  and/or a reflectivity map. When a reflectivity map is present it drives intensity;
  otherwise the previous material-restitution behavior is kept.

  Usage

Add a reflectivity_map attribute to a look in the scenario XML:

<look name="seabed" ... reflectivity_map="textures/seabed_reflectivity.png"/>