Remove support for DMA-BUF flags

They are never used in practice, which makes all of our flag
handling effectively dead code. Also, APIs such as KMS don't
provide a good way to deal with the flags. Let's just fail the
DMA-BUF import when clients provide flags.

include/wlr/render/gles2.h

@@ -30,7 +30,6 @@ struct wlr_gles2_texture_attribs {
 	GLenum target; /* either GL_TEXTURE_2D or GL_TEXTURE_EXTERNAL_OES */
 	GLuint tex;
 
-	bool inverted_y;
 	bool has_alpha;
 };