render/gles2: export gl fromat on texture attribs

When importing a wlr_texture as an external resource into Qt, Qt needs
to know its pixel format. This is similar to the usage scenario of the
format field in wlr_vk_image_attribs. Although it is possible to query
GL_TEXTURE_INTERNAL_FORMAT through the OpenGL API, it is not the same
value as the format that was originally passed to OpenGL when the
wlr_texture was created. The internal_format and format do not have a
one-to-one correspondence, and the correct original format cannot be
retrieved from outside.

include/render/gles2.h

@@ -124,6 +124,7 @@ struct wlr_gles2_texture {
 	// case.
 	GLuint tex;
 	GLuint fbo;
+	GLint format;
 
 	bool has_alpha;