Bug 1643832 - Add Pref flag to enable shader precaching at startup. r=jnicol

Differential Revision: https://phabricator.services.mozilla.com/D78580

gfx/webrender_bindings/RenderThread.cpp

@@ -926,7 +926,9 @@ void RenderThread::MaybeEnableGLDebugMessage(gl::GLContext* aGLContext) {
 WebRenderShaders::WebRenderShaders(gl::GLContext* gl,
                                    WebRenderProgramCache* programCache) {
   mGL = gl;
-  mShaders = wr_shaders_new(gl, programCache ? programCache->Raw() : nullptr);
+  mShaders =
+      wr_shaders_new(gl, programCache ? programCache->Raw() : nullptr,
+                     StaticPrefs::gfx_webrender_precache_shaders_AtStartup());
 }
 
 WebRenderShaders::~WebRenderShaders() {

gfx/webrender_bindings/src/bindings.rs

@@ -3749,10 +3749,11 @@ pub unsafe extern "C" fn wr_device_delete(device: *mut Device) {
 pub extern "C" fn wr_shaders_new(
     gl_context: *mut c_void,
     program_cache: Option<&mut WrProgramCache>,
+    precache_shaders: bool,
 ) -> *mut WrShaders {
     let mut device = wr_device_new(gl_context, program_cache);
 
-    let precache_flags = if env_var_to_bool("MOZ_WR_PRECACHE_SHADERS") {
+    let precache_flags = if precache_shaders || env_var_to_bool("MOZ_WR_PRECACHE_SHADERS") {
         ShaderPrecacheFlags::FULL_COMPILE
     } else {
         ShaderPrecacheFlags::ASYNC_COMPILE

modules/libpref/init/StaticPrefList.yaml

@@ -4274,6 +4274,11 @@
   value: true
   mirror: once
 
+- name: gfx.webrender.precache-shaders
+  type: bool
+  value: false
+  mirror: once
+
 # When gl debug message is a high severity message, forwward it to gfx critical
 # note.
 - name: gfx.webrender.gl-debug-message-critical-note