tweak renderer API

resources/shaders/source/font.frag.glsl

@@ -0,0 +1,21 @@
+#version 450
+
+layout(set = 3, binding = 0) uniform Viewport {
+    ivec2 view;
+};
+layout(set = 2, binding = 0) uniform sampler2D tx;
+
+layout(location = 0) in vec2 uv;
+layout(location = 1) in vec4 color;
+
+layout(location = 0) out vec4 fragColor;
+
+void main()
+{
+    ivec2 ts = textureSize(tx, 0);
+    vec2 fts = vec2(float(ts.x), float(ts.y));
+    vec2 real_uv = uv / fts;
+
+    vec4 opacity = texture(tx, real_uv);
+    fragColor = vec4(color.rgb, color.a*opacity.r);
+}