[RenderEngine] Cleanup Description.

Make Description a struct.

Minor: Use mat4 instead of mat3.

BUG: 112585051
Test: Build, flash and boot
Change-Id: I8f4b8ac1fd847239b42af9685ba0cddb15f0fac7
diff --git a/services/surfaceflinger/RenderEngine/gl/ProgramCache.h b/services/surfaceflinger/RenderEngine/gl/ProgramCache.h
index ea77a2d..47963eb 100644
--- a/services/surfaceflinger/RenderEngine/gl/ProgramCache.h
+++ b/services/surfaceflinger/RenderEngine/gl/ProgramCache.h
@@ -29,7 +29,7 @@
 
 namespace renderengine {
 
-class Description;
+struct Description;
 
 namespace gl {