Remove usages of texture2D and other FL0 features that are obsolete due to FL0 now going through spirv-cross.

This is an early upload of https://github.com/google/filament/pull/9867 and necessary material changes

Original PR commits:
FL0 materials are always compile via spirv-cross
postfx materials now support sRGB emulation in FL0

PiperOrigin-RevId: 899262890
Change-Id: I52c002d60790d84269674c1f0d0c5f7d6a58000c

src/experimental/filament/assets/unlit_ui.mat

@@ -36,7 +36,7 @@ fragment {
         prepareMaterial(material);
         vec2 uv = getUV0();
         uv.y = 1.0 - uv.y;
-        vec4 tex_color = texture2D(materialParams_glyph, uv);
+        vec4 tex_color = texture(materialParams_glyph, uv);
         material.baseColor = getColor() * tex_color;
         material.baseColor.rgb *= material.baseColor.a;
     }