gpu,gpu/shaders: [compute] decode sRGB texels in shader when EXT_sRGB is missing

This change avoids the hard dependency on GPU support for sRGB encoded
textures in the compute renderer.

With this change and the previously added CPU fallback, Gio no longer
rely on any GPU functionality outside the OpenGL ES 2.0 level.

Fixes gio#49
Fixes gio#154
Fixes gio#97
Fixes gio#36
Fixes gio#172

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

gpu/shaders/material.frag

@@ -10,6 +10,12 @@ layout(location = 0) in vec2 vUV;
 
 layout(location = 0) out vec4 fragColor;
 
+layout(binding=0) uniform Color {
+	// If emulateSRGB is set (!= 0), the input texels are sRGB encoded. We save the
+	// conversion step below, at the cost of texture filtering in sRGB space.
+	float emulateSRGB;
+};
+
 vec3 RGBtosRGB(vec3 rgb) {
 	bvec3 cutoff = greaterThanEqual(rgb, vec3(0.0031308));
 	vec3 below = vec3(12.92)*rgb;
@@ -19,6 +25,8 @@ vec3 RGBtosRGB(vec3 rgb) {
 
 void main() {
 	vec4 texel = texture(tex, vUV);
-	texel.rgb = RGBtosRGB(texel.rgb);
+	if (emulateSRGB == 0.0) {
+		texel.rgb = RGBtosRGB(texel.rgb);
+	}
 	fragColor = texel;
 }