render/vulkan: allow rendering to non-8-bit buffers

This is implemented by a two-subpass rendering scheme; the first
subpass draws (and blends) onto a linear R16G16B16A16_SFLOAT buffer,
while the second subpass performs linear->srgb conversion, writing
onto the actual output buffer.

render/vulkan/shaders/output.frag

@@ -0,0 +1,21 @@
+#version 450
+
+layout (input_attachment_index = 0, binding = 0) uniform subpassInput in_color;
+
+layout(location = 0) in vec2 uv;
+layout(location = 0) out vec4 out_color;
+
+float linear_to_srgb(float x) {
+	return max(min(x * 12.92, 0.04045), 1.055 * pow(x, 1. / 2.4) - 0.055);
+}
+
+void main() {
+	vec4 val = subpassLoad(in_color).rgba;
+	out_color = vec4(
+		linear_to_srgb(val.r),
+		linear_to_srgb(val.g),
+		linear_to_srgb(val.b),
+		val.a
+	);
+}
+