Amazingly, the size field in PixelBufferDescriptor was often totally
incorrect for Java-based clients. The reason we did not notice: OpenGL
often consumes a pointer to image data without consuming a byte count
(e.g. glTexImage2D).
OpenGL infers size from dimensions + format, but Vulkan does not. This
caused texture corruption with Vulkan on Android.
This fix follows a pattern used in other places such as
FRenderer::readPixels.
6110809d23