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/meson.build

@@ -2,6 +2,7 @@ vulkan_shaders_src = [
 	'common.vert',
 	'texture.frag',
 	'quad.frag',
+	'output.frag',
 ]
 
 vulkan_shaders = []