render/gles2: reword wlr_gles2_texture comments

These comments were a bit misleading:

- "GL_TEXTURE_2D == mutable": not really, imported non-external-only
  DMA-BUFs would also use this target, but are not mutable.
- "Only affects target == GL_TEXTURE_2D": same here.
- "If imported from a wlr_buffer": not really, would be NULL if
  imported from a shm wlr_buffer.

Adjust these comments to better reflect reality and adjust the check
in gles2_texture_update_from_buffer().

include/render/gles2.h

@@ -118,19 +118,14 @@ struct wlr_gles2_texture {
 	struct wlr_gles2_renderer *renderer;
 	struct wl_list link; // wlr_gles2_renderer.textures
 
-	// Basically:
-	//   GL_TEXTURE_2D == mutable
-	//   GL_TEXTURE_EXTERNAL_OES == immutable
 	GLenum target;
 	bool owns_tex;
 	GLuint tex;
 
 	bool has_alpha;
 
-	// Only affects target == GL_TEXTURE_2D
-	uint32_t drm_format; // used to interpret upload data
-	// If imported from a wlr_buffer
-	struct wlr_gles2_buffer *buffer;
+	uint32_t drm_format; // for mutable textures only, used to interpret upload data
+	struct wlr_gles2_buffer *buffer; // for DMA-BUF imports only
 };
 
 struct wlr_gles2_render_pass {

render/gles2/texture.c

@@ -32,7 +32,7 @@ static bool gles2_texture_update_from_buffer(struct wlr_texture *wlr_texture,
 		struct wlr_buffer *buffer, const pixman_region32_t *damage) {
 	struct wlr_gles2_texture *texture = gles2_get_texture(wlr_texture);
 
-	if (texture->target != GL_TEXTURE_2D || texture->buffer) {
+	if (texture->drm_format == DRM_FORMAT_INVALID) {
 		return false;
 	}