Fix build error with -O2

../render/gles2/renderer.c: In function ‘gles2_render_texture_with_matrix’:
../render/gles2/renderer.c:140:2: error: ‘target’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  glBindTexture(target, tex_id);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../render/gles2/renderer.c:145:2: error: ‘prog’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  glUseProgram(prog);

render/gles2/renderer.c

@@ -111,8 +111,8 @@ static bool gles2_render_texture_with_matrix(struct wlr_renderer *wlr_renderer,
 	struct wlr_gles2_texture *texture =
 		gles2_get_texture_in_context(wlr_texture);
 
-	GLuint prog;
-	GLenum target;
+	GLuint prog = 0;
+	GLenum target = 0;
 	switch (texture->type) {
 	case WLR_GLES2_TEXTURE_GLTEX:
 	case WLR_GLES2_TEXTURE_WL_DRM_GL: