don't reallocate the mesh each time we use it

the Mesh object can be part of each Layer (at least currently).
also reworked the Mesh code a bit to make it easier to access
the vertex data.

Change-Id: I0490851ba898f0aa2e55b62958dcd8bdb535e98b
diff --git a/services/surfaceflinger/RenderEngine/GLES20RenderEngine.cpp b/services/surfaceflinger/RenderEngine/GLES20RenderEngine.cpp
index 6786065..9be12bf 100644
--- a/services/surfaceflinger/RenderEngine/GLES20RenderEngine.cpp
+++ b/services/surfaceflinger/RenderEngine/GLES20RenderEngine.cpp
@@ -168,7 +168,7 @@
             mesh.getVertexSize(),
             GL_FLOAT, GL_FALSE,
             mesh.getByteStride(),
-            mesh.getVertices());
+            mesh.getPositions());
 
     glDrawArrays(mesh.getPrimitive(), 0, mesh.getVertexCount());
 }
@@ -190,7 +190,7 @@
             mesh.getVertexSize(),
             GL_FLOAT, GL_FALSE,
             mesh.getByteStride(),
-            mesh.getVertices());
+            mesh.getPositions());
 
     glDrawArrays(mesh.getPrimitive(), 0, mesh.getVertexCount());