mesa: rework viewport/scissor initialization code

The first time a context is bound to a drawable, the viewport and scissor
bounds are initialized to the buffer's size.  This is actually a bit tricky.

A new _mesa_check_init_viewport() function is called in several places
to check if the viewport has been initialized.  We also use a new
ctx->ViewportInitialized flag instead of the overloaded
ctx->FirstTimeCurrent flag.

src/mesa/main/context.c

@@ -1255,6 +1255,24 @@ initialize_framebuffer_size(GLcontext *ctx, GLframebuffer *fb)
 }
 
 
+/**
+ * Check if the viewport/scissor size has not yet been initialized.
+ * Initialize the size if the given width and height are non-zero.
+ */
+void
+_mesa_check_init_viewport(GLcontext *ctx, GLuint width, GLuint height)
+{
+   if (!ctx->ViewportInitialized && width > 0 && height > 0) {
+      /* Note: set flag here, before calling _mesa_set_viewport(), to prevent
+       * potential infinite recursion.
+       */
+      ctx->ViewportInitialized = GL_TRUE;
+      _mesa_set_viewport(ctx, 0, 0, width, height);
+      _mesa_set_scissor(ctx, 0, 0, width, height);
+   }
+}
+
+
 /**
  * Bind the given context to the given drawBuffer and readBuffer and
  * make it the current context for the calling thread.
@@ -1372,25 +1390,24 @@ _mesa_make_current( GLcontext *newCtx, GLframebuffer *drawBuffer,
          ASSERT(drawBuffer->Height > 0);
 #endif
 
-         if (newCtx->FirstTimeCurrent) {
-            /* set initial viewport and scissor size now */
-            _mesa_set_viewport(newCtx, 0, 0,
-                               drawBuffer->Width, drawBuffer->Height);
-	    _mesa_set_scissor(newCtx, 0, 0,
-			      drawBuffer->Width, drawBuffer->Height );
-            check_context_limits(newCtx);
+         if (drawBuffer) {
+            _mesa_check_init_viewport(newCtx,
+                                      drawBuffer->Width, drawBuffer->Height);
          }
       }
 
-      /* We can use this to help debug user's problems.  Tell them to set
-       * the MESA_INFO env variable before running their app.  Then the
-       * first time each context is made current we'll print some useful
-       * information.
-       */
       if (newCtx->FirstTimeCurrent) {
+         check_context_limits(newCtx);
+
+         /* We can use this to help debug user's problems.  Tell them to set
+          * the MESA_INFO env variable before running their app.  Then the
+          * first time each context is made current we'll print some useful
+          * information.
+          */
 	 if (_mesa_getenv("MESA_INFO")) {
 	    _mesa_print_info();
 	 }
+
 	 newCtx->FirstTimeCurrent = GL_FALSE;
       }
    }