render/gles2: assert that GL_OES_EGL_image_external is supported

The target is set to GL_TEXTURE_EXTERNAL_OES when
EGL_EXT_image_dma_buf_import_modifiers [1] returns an external_only
flag. That spec states:

> In order to support imports for the GL_TEXTURE_EXTERNAL_OES target, a
> compatible OpenGL ES implementation supporting GL_OES_EGL_image_external
> must be present.

Fail hearder when a driver doesn't follow the spec instead of
skipping rendering.

See [2].

[1]: https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import_modifiers.txt
[2]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/3631#note_1584343

render/gles2/renderer.c

@@ -267,13 +267,10 @@ static bool gles2_render_subtexture_with_matrix(
 		}
 		break;
 	case GL_TEXTURE_EXTERNAL_OES:
+		// EGL_EXT_image_dma_buf_import_modifiers requires
+		// GL_OES_EGL_image_external
+		assert(renderer->exts.OES_egl_image_external);
 		shader = &renderer->shaders.tex_ext;
-
-		if (!renderer->exts.OES_egl_image_external) {
-			wlr_log(WLR_ERROR, "Failed to render texture: "
-				"GL_TEXTURE_EXTERNAL_OES not supported");
-			return false;
-		}
 		break;
 	default:
 		abort();