gl-renderer: allow importing fourth dmabuf plane

EGL_EXT_image_dma_buf_import_modifiers supports importing upto four dmabuf
planes into an EGLImage.

v2: correct PLANE3_PITCH token (Daniel Stone)

Signed-off-by: Varad Gautam <varad.gautam@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>