vulkan: add support for blend buffers

src/gfx_apis/vulkan/shaders/out.frag

@@ -0,0 +1,17 @@
+#version 450
+#extension GL_EXT_samplerless_texture_functions : require
+
+#include "frag_spec_const.glsl"
+#include "transfer_functions.glsl"
+#include "out.common.glsl"
+
+layout(set = 0, binding = 0) uniform texture2D in_color;
+layout(location = 0) out vec4 out_color;
+
+void main() {
+	vec4 c = texelFetch(in_color, ivec2(gl_FragCoord.xy), 0);
+	c.rgb /= mix(c.a, 1.0, c.a == 0.0);
+	c.rgb = oetf_srgb(c.rgb);
+	c.rgb *= c.a;
+	out_color = c;
+}