Add option to enable HDR rendering in 2D

This is needed to allow 2D to fully make use of 3D effects (e.g. glow), and can be used to substantially improve quality of 2D rendering at the cost of performance

Additionally, the 2D rendering pipeline is done in linear space (we skip linear_to_srgb conversion in 3D tonemapping) so the entire Viewport can be kept linear.
This is necessary for proper HDR screen support in the future.

servers/rendering/renderer_rd/shaders/blit.glsl

@@ -45,7 +45,7 @@ layout(push_constant, std140) uniform Pos {
 	float upscale;
 	float aspect_ratio;
 	uint layer;
-	uint pad1;
+	bool convert_to_srgb;
 }
 data;
 
@@ -59,6 +59,13 @@ layout(binding = 0) uniform sampler2DArray src_rt;
 layout(binding = 0) uniform sampler2D src_rt;
 #endif
 
+vec3 linear_to_srgb(vec3 color) {
+	// If going to srgb, clamp from 0 to 1.
+	color = clamp(color, vec3(0.0), vec3(1.0));
+	const vec3 a = vec3(0.055f);
+	return mix((vec3(1.0f) + a) * pow(color.rgb, vec3(1.0f / 2.4f)) - a, 12.92f * color.rgb, lessThan(color.rgb, vec3(0.0031308f)));
+}
+
 void main() {
 #ifdef APPLY_LENS_DISTORTION
 	vec2 coords = uv * 2.0 - 1.0;
@@ -94,4 +101,8 @@ void main() {
 #else
 	color = texture(src_rt, uv);
 #endif
+
+	if (data.convert_to_srgb) {
+		color.rgb = linear_to_srgb(color.rgb); // Regular linear -> SRGB conversion.
+	}
 }