Improve texture filtering by doing it in gamma space (#7311)

* Closes https://github.com/emilk/egui/pull/5839

This makes some transparent images look a lot nicer when blended:


![image](https://github.com/user-attachments/assets/7f370aaf-886a-423c-8391-c378849b63ca)

Cursive text will also look nicer.

This unfortunately changes the contract of what
`register_native_texture` expects

---------

Co-authored-by: Adrian Blumer <blumer.adrian@gmail.com>

crates/egui_glow/src/shader/fragment.glsl

@@ -43,25 +43,8 @@ vec3 dither_interleaved(vec3 rgb, float levels) {
     return rgb + noise / (levels - 1.0);
 }
 
-// 0-1 sRGB gamma  from  0-1 linear
-vec3 srgb_gamma_from_linear(vec3 rgb) {
-    bvec3 cutoff = lessThan(rgb, vec3(0.0031308));
-    vec3 lower = rgb * vec3(12.92);
-    vec3 higher = vec3(1.055) * pow(rgb, vec3(1.0 / 2.4)) - vec3(0.055);
-    return mix(higher, lower, vec3(cutoff));
-}
-
-// 0-1 sRGBA gamma  from  0-1 linear
-vec4 srgba_gamma_from_linear(vec4 rgba) {
-    return vec4(srgb_gamma_from_linear(rgba.rgb), rgba.a);
-}
-
 void main() {
-#if SRGB_TEXTURES
-    vec4 texture_in_gamma = srgba_gamma_from_linear(texture2D(u_sampler, v_tc));
-#else
     vec4 texture_in_gamma = texture2D(u_sampler, v_tc);
-#endif
 
     // We multiply the colors in gamma space, because that's the only way to get text to look right.
     vec4 frag_color_gamma = v_rgba_in_gamma * texture_in_gamma;