Make the debug interface a GrGLTestInterface subclass.

The debug interface subsumes the GrDebugGL singleton which allows multiple instances.

Fixes a few issues that existed before this CL (pack alignment wasn't tracked or respected, and we weren't initializing a texture id to 0 in one GM). Apparently this is not currently run on the bots.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1812323002

Review URL: https://codereview.chromium.org/1812323002
diff --git a/gm/rectangletexture.cpp b/gm/rectangletexture.cpp
index e00afe9..43d008a 100644
--- a/gm/rectangletexture.cpp
+++ b/gm/rectangletexture.cpp
@@ -87,7 +87,7 @@
 // Useful for debugging whether errors result from use of RECTANGLE
 // #define TARGET GR_GL_TEXTURE_2D
 #define TARGET GR_GL_TEXTURE_RECTANGLE
-        GrGLuint id;
+        GrGLuint id = 0;
         GR_GL_CALL(gl, GenTextures(1, &id));
         GR_GL_CALL(gl, BindTexture(TARGET, id));
         GR_GL_CALL(gl, TexParameteri(TARGET, GR_GL_TEXTURE_MAG_FILTER,