[gl] Clear the current_target cache when binding a framebuffer in creation.

Failing to do so meant that we might not re-bind the framebuffer to
the correct one if we didn't follow up surface creation with drawing
to it.

Fixes many testcases.  Thanks to T. Zachary Laine for tracking down
that the current_target was the source of these issues.

src/cairo-gl-surface.c

@@ -503,6 +503,7 @@ _cairo_gl_surface_create_scratch (cairo_gl_context_t   *ctx,
 			       GL_TEXTURE_2D,
 			       surface->tex,
 			       0);
+    ctx->current_target = NULL;
 
     while ((err = glGetError ())) {
 	fprintf (stderr, "GL error in surface create: 0x%08x\n", err);