Depth
Packs a 24-bit normalized depth-buffer value into the red, green and blue channels of an RGBA8 color attachment so it can be retrieved with glReadPixels(GL_RGBA, GL_UNSIGNED_BYTE). WebGL1 / GLES2 do not allow reading depth-component textures with glReadPixels, so the engine renders the pick-pass depth texture through this program onto a colour attachment, then unpacks the bytes on the CPU.
Encoding: red carries the high byte, green the middle byte, blue the low byte. The 24-bit pack matches DEPTH24's source precision on WebGL2 / GLES3+ — see earth.worldwind.draw.DrawContext.ensurePickFramebuffer. WebGL1 / GLES2 depth attachments are at most 16-bit so the low byte is zero there; the unpack is identical either way.
The vertex shader expects a unit-square triangle strip at attribute 0 (see earth.worldwind.draw.DrawContext.unitSquareBuffer) and rasterises the full output framebuffer. loadUvScale must be called every blit with viewport.size / depthTexture.size to clamp vUV to the rendered region of the (oversized) depth texture.