Fix D3D11 prematurely disposing swapchain framebuffer texture

[frenzibyte]

This is split from #363 to revive the PR while reducing the scope of the changes, and also includes a fix for the original changes as not disposing the texture at all results in a fatal error when calling ResizeBuffers on the next swapchain resize call.

[bdach]

Note for posterity: if this is applied on top of #485, then the two changes will not combine correctly despite not having merge conflicts. The undisposed swapchain framebuffer texture must be disposed before the swapchain is recreated to enable/disable tearing - otherwise mutating the AllowTearing flag will result in a crash due to an access violation.

Just figure it's better to mention it, since we're using both patches in our fork and just stumbled on this ourselves.