Implement async shader compilation plus caching for GL ES 3

Async. compilation via ubershader is currently available in the scene and particles shaders only.

Bonus:
- Use `#if defined()` syntax for not true conditionals, so they don't unnecessarily take a bit in the version flagset.
- Remove unused `ENABLE_CLIP_ALPHA` from scene shader.
- Remove unused `PARTICLES_COPY` from the particles shader.
- Remove unused uniform related code.
- Shader language/compiler: use ordered hash maps for deterministic code generation (needed for caching).

drivers/gles3/rasterizer_gles3.cpp

@@ -207,6 +207,7 @@ void RasterizerGLES3::begin_frame(double frame_step) {
 	storage->frame.time[2] = Math::fmod(time_total, 900);
 	storage->frame.time[3] = Math::fmod(time_total, 60);
 	storage->frame.count++;
+	storage->frame.shader_compiles_started = 0;
 	storage->frame.delta = frame_step;
 
 	storage->update_dirty_resources();
@@ -214,6 +215,8 @@ void RasterizerGLES3::begin_frame(double frame_step) {
 	storage->info.render_final = storage->info.render;
 	storage->info.render.reset();
 
+	ShaderGLES3::current_frame = storage->frame.count;
+
 	scene->iteration();
 }
 
@@ -410,6 +413,8 @@ void RasterizerGLES3::end_frame(bool p_swap_buffers) {
 		}
 	}
 
+	ShaderGLES3::advance_async_shaders_compilation();
+
 	if (p_swap_buffers) {
 		OS::get_singleton()->swap_buffers();
 	} else {
@@ -487,6 +492,8 @@ RasterizerGLES3::RasterizerGLES3() {
 
 	time_total = 0;
 	time_scale = 1;
+
+	ShaderGLES3::compiles_started_this_frame = &storage->frame.shader_compiles_started;
 }
 
 RasterizerGLES3::~RasterizerGLES3() {