"paint" example leaks memory

[bvssvni]

Might be a memory leak in Gfx.

Since updating textures requires access to Encoder in gfx_graphics, the only way to "update" a texture, for now, is by creating a new texture. This line replaces the old texture with the new one.

The "paint" example has a memory leak, which I thought this call in piston_window should clean up.

cc @kvark

[kvark]

This is possible, perhaps related to gfx-rs/gfx#1054