vc4: Claim ARB_fbo.

This gets a ton of piglit working that crashes in waffle context
management stuff otherwise.  Actually supporting mismatched FB sizes is at
best going to require some more load/store generals for color buffers, but
if I can't manage to do that I'll want to just have state_tracker reject
those FBOs as unsupported, rather than deny GL 2.1.

src/gallium/drivers/vc4/vc4_screen.c

@@ -108,8 +108,10 @@ vc4_screen_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
         case PIPE_CAP_TGSI_FS_COORD_PIXEL_CENTER_HALF_INTEGER:
                 return 1;
 
-                /* Unsupported features. */
         case PIPE_CAP_MIXED_FRAMEBUFFER_SIZES:
+                return 1;
+
+                /* Unsupported features. */
         case PIPE_CAP_ANISOTROPIC_FILTER:
         case PIPE_CAP_TEXTURE_BUFFER_OBJECTS:
         case PIPE_CAP_CUBE_MAP_ARRAY: