ensureGaussianPassFramebuffer

Reduced-resolution Gaussian-splat pass target: RGBA color (LINEAR, for the composite upsample) + 24-bit depth texture for the terrain occlusion prepass. Grows monotonically like the pick framebuffer; callers render into the lower-left width x height region and sample it back via a texture-scale uniform. Requires sized texture formats — callers gate on earth.worldwind.util.kgl.Kgl.supportsSizedTextureFormats and fall back to direct drawing otherwise. Returns null if the framebuffer cannot be completed.