#14714 Release offscreen FBO OpenGL resources on context loss/recreation

[magnesj]

Fixes #14714.

Problem

caf::Viewer relied on a QOpenGLContext::aboutToBeDestroyed connection to release the offscreen FBO's OpenGL resources, but the connection was dead code:

• Slot ordering: OpenGLWidget's own aboutToBeDestroyed slot ran first and nulled the cvf OpenGL context, so deleteFboOpenGLResources() became a no-op.
• One-shot connection: the connection was only ever made once (onWidgetOpenGLReady()), so it did not fire again after a context was recreated (e.g. when a dock widget is reparented across top-level windows, which Qt::AA_ShareOpenGLContexts enables).

As a result m_offscreenFbo kept OpenGL ids belonging to a destroyed context. FramebufferObject::applyOpenGL() only regenerates ids that are still 0, so a stale non-zero id caused glBindFramebuffer to silently create a fresh, attachment-less framebuffer. Attachments are only reapplied when the viewport size changes, so a view re-docked at an unchanged size stayed incomplete and rendered black — this is the reproducible symptom in #14708 (tiling two ensemble contour maps at the same size blacks one out), and likely the underlying cause of the RHEL8 completely-black-view reports.

Fix

Added explicit, well-documented lifecycle hooks to cvfqt::OpenGLWidget:

• onWidgetOpenGLAboutToBeShutdown() — called before context teardown, with the old context still current.
• onWidgetOpenGLReinitialized() — called from initializeGL() on reinit, with the new context current (replaces the bare update() call).

caf::Viewer overrides both to release/reset the offscreen FBO's cached OpenGL resources, so they get rebuilt correctly in the new context regardless of which lifecycle event actually fires on a given platform.