gpu: [compute] pre-transform images before rendering

We're about to change the last stage of the compute pipeline to only
accept images, not sampled textures. This change prepares materials
for pixel-aligned image copying by pre-rendering images to a texture,
applying transforms.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/material.frag

@@ -0,0 +1,24 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision mediump float;
+
+layout(binding = 0) uniform sampler2D tex;
+
+layout(location = 0) in vec2 vUV;
+
+layout(location = 0) out vec4 fragColor;
+
+vec3 RGBtosRGB(vec3 rgb) {
+	bvec3 cutoff = greaterThanEqual(rgb, vec3(0.0031308));
+	vec3 below = vec3(12.92)*rgb;
+	vec3 above = vec3(1.055)*pow(rgb, vec3(0.41666)) - vec3(0.055);
+	return mix(below, above, cutoff);
+}
+
+void main() {
+	vec4 texel = texture(tex, vUV);
+	texel.rgb = RGBtosRGB(texel.rgb);
+	fragColor = texel;
+}