libgui|GLTarget: Reconfiguring an existing GLTarget instance
libgui|GLTexture: Setting up a depth+stencil texture
libgui|GLState|GLUniform: Prepare for deletion of assets

GLState must be aware that a render target might be deleted while
it is the current target.

GLUniform must be aware that a texture might be deleted while it
is the current value of a sampler uniform.
libgui|Canvas: Working on manual frame buffer mode

Canvas needs to support rendering depth values to a texture.
To accomplish this, the FBO must be configured with suitable
attachments.

In the manual frame buffer mode, two textures are used for color
and depth+stencil values.

Todo: Multisampling must be accounted for separately.
Renderer|FX: Use depth buffer to occlude lens flares

Work in progress…