Use partial texture uploads for shm buffers

Raster-based Wayland clients running against a QtWayland compositor will normally reuse the same buffer, but SharedMemoryBuffer::toOpenGlTexture() will always re-upload the whole buffer using glTexImage2D(), even if only a small part of the buffer has been damaged.

In order to improve performance, we should use glTexSubImage2D() to update only the part of the texture that has been changed. Uploading a rectangular region of a raster image is done by setting the stride with glPixelStorei(GL_PACK_ROW_LENGTH, stride). Note that this is not available in GLES 2, where we would have to upload a sub-image that has the full width of the buffer.

This will also require some book-keeping to keep track of the dirty region.