mesa: fix inital value for new renderbuffer

EXT_framebuffer_object bspec says:

    Get Value                          Type    Get Command 		Initial Value
    -------------------------------    ------  -----------      	-----------
    RENDERBUFFER_INTERNAL_FORMAT_EXT   Z+     GetRenderbufferParameterivEXT  RGBA

NOTE: this is a candidate for the 7.11 branch

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/renderbuffer.c

@@ -2011,7 +2011,7 @@ _mesa_init_renderbuffer(struct gl_renderbuffer *rb, GLuint name)
 
    rb->Width = 0;
    rb->Height = 0;
-   rb->InternalFormat = GL_NONE;
+   rb->InternalFormat = GL_RGBA;
    rb->Format = MESA_FORMAT_NONE;
 
    rb->DataType = GL_NONE;