Merge #1476

1476: [ll] Frames and encoders for render r=kvark a=Bastacyclop

Working on #1466.
This is an untested draft, I hope there are not too many mistakes.
The interesting stuff is in [core/pool](https://github.com/gfx-rs/gfx/compare/ll...Bastacyclop:ll?expand=1#diff-a8c8cf11be50bbdb6bab730b503bfe00), [render/device](https://github.com/gfx-rs/gfx/compare/ll...Bastacyclop:ll?expand=1#diff-a505eeca0919c44fc9d868f9a1cbdfe3) and more importantly [render/encoder](https://github.com/gfx-rs/gfx/compare/ll...Bastacyclop:ll?expand=1#diff-e76accb5ddad96024c8ebca0ad05ee68) and [render/lib](https://github.com/gfx-rs/gfx/compare/ll...Bastacyclop:ll?expand=1#diff-be2c920c08146924af4d1983d6894d63).
My next step will be to test it in a copy of the quad example.

You can acquire `encoder::Scope`s from which you can get actual `Encoder`s. An `encoder::Scope` is basically a more convenient command buffer pool. So there are two reasons to use multiple scopes: to use multiple encoders at the same time (multithreading mostly) and to release all the acquired encoders independently (different encoding scopes). Maybe scope is not a good name and I should name it simply pool instead.

Cargo.toml

@@ -9,13 +9,14 @@ members = [
     "src/backend/vulkan",
     "src/core",
     #"src/macros",
-    #"src/render",
+    "src/render",
     #"src/support",
     #"src/window/dxgi",
     #"src/window/glfw",
     #"src/window/glutin",
     #"src/window/metal",
     #"src/window/sdl",
     #"examples",
-    "examples/core/quad"
+    "examples/core/quad",
+    "examples/render/quad_render",
 ]

examples/core/quad/main.rs

@@ -77,7 +77,7 @@ fn main() {
     let mut queue = graphics_queues.remove(0);
     let swap_config = SwapchainConfig::new()
         .with_color::<ColorFormat>();
-    let mut swap_chain = surface.build_swapchain(swap_config, &queue);
+    let (mut swap_chain, backbuffers) = surface.build_swapchain(swap_config, &queue);
 
     // Setup renderpass and pipeline
     // dx12 runtime shader compilation
@@ -220,7 +220,7 @@ fn main() {
     let set1 = sampler_pool.allocate_sets(&[&set1_layout]);
 
     // Framebuffer and render target creation
-    let frame_rtvs = swap_chain.get_backbuffers().iter().map(|bb| {
+    let frame_rtvs = backbuffers.iter().map(|bb| {
         device.view_image_as_render_target(&bb.color, ColorFormat::get_format(), (0..1, 0..1)).unwrap()
     }).collect::<Vec<_>>();
 
@@ -401,7 +401,7 @@ fn main() {
         let submit = {
             let mut cmd_buffer = graphics_pool.acquire_command_buffer();
 
-            let rtv = &swap_chain.get_backbuffers()[frame.id()].color;
+            let rtv = &backbuffers[frame.id()].color;
             if do_barriers {
                 let rtv_target_barrier = m::Barrier::Image {
                     states: (i::Access::empty(), i::ImageLayout::Undefined) ..

examples/render/quad_render/Cargo.toml

@@ -0,0 +1,53 @@
+[package]
+name = "quad_render"
+version = "0.1.0"
+publish = false
+workspace = "../../.."
+
+[features]
+default = ["vulkan"]
+#metal = ["gfx_device_metal", "gfx_window_metal"]
+gl = ["glutin", "gfx_backend_gl"]
+#dx11 = ["gfx_device_dx11", "gfx_window_dxgi"]
+#dx12 = ["gfx_device_dx12", "gfx_window_dxgi"]
+vulkan = ["gfx_backend_vulkan"]
+unstable = []
+
+[[bin]]
+name = "quad_render"
+path = "main.rs"
+
+[dependencies]
+env_logger = "0.4"
+glutin = { version = "0.9", optional = true }
+image = "0.15"
+log = "0.3"
+winit = "0.7"
+gfx_core = { path = "../../../src/core", version = "0.10" }
+gfx = { path = "../../../src/render", version = "0.17" }
+
+[dependencies.gfx_backend_gl]
+path = "../../../src/backend/gl"
+version = "0.1"
+optional = true
+
+[dependencies.gfx_backend_vulkan]
+path = "../../../src/backend/vulkan"
+version = "0.1"
+optional = true
+
+#[dependencies.gfx_device_metal]
+#path = "../../../src/backend/metal"
+#version = "0.3"
+#optional = true
+
+#[dependencies.gfx_window_metal]
+#path = "../../../src/window/metal"
+#version = "0.4"
+#optional = true
+
+#[target.'cfg(windows)'.dependencies]
+#gfx_device_dx11 = { path = "../../../src/backend/dx11", version = "0.6", optional = true }
+#gfx_device_dx12 = { path = "../../../src/backend/dx12", version = "0.1", optional = true }
+#gfx_window_dxgi = { path = "../../../src/window/dxgi", version = "0.9", optional = true }
+