Initial vulkan context and immediate drawer.

Extremely rough, just checking in so DrChat can snoop.

src/xenia/ui/vulkan/shaders/immediate.vert

@@ -0,0 +1,21 @@
+#version 450 core
+precision highp float;
+
+layout(push_constant) uniform PushConstants {
+  mat4 projection_matrix;
+  int restrict_texture_samples;
+} push_constants;
+
+layout(location = 0) in vec2 in_pos;
+layout(location = 1) in vec2 in_uv;
+layout(location = 2) in vec4 in_color;
+
+layout(location = 0) out vec2 vtx_uv;
+layout(location = 1) out vec4 vtx_color;
+
+void main() {
+  gl_Position = push_constants.projection_matrix * vec4(in_pos.xy, 0.0, 1.0);
+  gl_Position.y = -gl_Position.y;
+  vtx_uv = in_uv;
+  vtx_color = in_color;
+}