[cogl] cache the viewport width and height

This avoids some calls to glGetFloatv, which have at least proven to be very
in-efficient in mesa at this point in time, since it always updates all derived
state even when it may not relate to the state being requested.

gl/cogl-context.c

@@ -92,6 +92,9 @@ cogl_create_context ()
 
   _context->in_begin_gl_block = FALSE;
 
+  _context->viewport_width = 0;
+  _context->viewport_height = 0;
+
   _context->pf_glGenRenderbuffersEXT = NULL;
   _context->pf_glBindRenderbufferEXT = NULL;
   _context->pf_glRenderbufferStorageEXT = NULL;

gl/cogl-context.h

@@ -112,6 +112,9 @@ typedef struct
 
   gboolean          in_begin_gl_block;
 
+  guint             viewport_width;
+  guint             viewport_height;
+
   /* Relying on glext.h to define these */
   COGL_PFNGLGENRENDERBUFFERSEXTPROC                pf_glGenRenderbuffersEXT;
   COGL_PFNGLDELETERENDERBUFFERSEXTPROC             pf_glDeleteRenderbuffersEXT;

gl/cogl-fbo.c

@@ -209,7 +209,7 @@ cogl_set_draw_buffer (CoglBufferTarget target, CoglHandle offscreen)
 	}
 
       /* Setup new viewport and matrices */
-      GE( glViewport (0, 0, fbo->width, fbo->height) );
+      cogl_viewport (fbo->width, fbo->height);
       _cogl_current_matrix_translate (-1.0f, -1.0f, 0.0f);
       _cogl_current_matrix_scale (2.0f / fbo->width, 2.0f / fbo->height, 1.0f);