Initial vulkan context and immediate drawer.

Extremely rough, just checking in so DrChat can snoop.

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

@@ -0,0 +1,23 @@
+#version 450 core
+precision highp float;
+
+layout(push_constant) uniform PushConstants {
+  mat4 projection_matrix;
+  int restrict_texture_samples;
+} push_constants;
+
+layout(set = 0, binding = 0) uniform sampler2D texture_sampler;
+
+layout(location = 0) in vec2 vtx_uv;
+layout(location = 1) in vec4 vtx_color;
+
+layout(location = 0) out vec4 out_color;
+
+void main() {
+  out_color = vtx_color;
+  if (push_constants.restrict_texture_samples == 0 || vtx_uv.x <= 1.0) {
+    vec4 tex_color = texture(texture_sampler, vtx_uv);
+    out_color *= tex_color;
+    // TODO(benvanik): microprofiler shadows.
+  }
+}