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/effects/tone_mapper.h

@@ -59,14 +59,23 @@ private:
 		TONEMAP_MODE_MAX
 	};
 
+	enum {
+		TONEMAP_FLAG_USE_BCS = (1 << 0),
+		TONEMAP_FLAG_USE_GLOW = (1 << 1),
+		TONEMAP_FLAG_USE_AUTO_EXPOSURE = (1 << 2),
+		TONEMAP_FLAG_USE_COLOR_CORRECTION = (1 << 3),
+		TONEMAP_FLAG_USE_FXAA = (1 << 4),
+		TONEMAP_FLAG_USE_DEBANDING = (1 << 5),
+		TONEMAP_FLAG_CONVERT_TO_SRGB = (1 << 6),
+	};
+
 	struct TonemapPushConstant {
 		float bcs[3]; // 12 - 12
-		uint32_t use_bcs; //  4 - 16
+		uint32_t flags; //  4 - 16
 
-		uint32_t use_glow; //  4 - 20
-		uint32_t use_auto_exposure; //  4 - 24
-		uint32_t use_color_correction; //  4 - 28
-		uint32_t tonemapper; //  4 - 32
+		float pixel_size[2]; //  8 - 24
+		uint32_t tonemapper; //  4 - 28
+		uint32_t pad; //  4 - 32
 
 		uint32_t glow_texture_size[2]; //  8 - 40
 		float glow_intensity; //  4 - 44
@@ -79,10 +88,6 @@ private:
 		float white; //  4 - 88
 		float auto_exposure_scale; //  4 - 92
 		float luminance_multiplier; //  4 - 96
-
-		float pixel_size[2]; //  8 - 104
-		uint32_t use_fxaa; //  4 - 108
-		uint32_t use_debanding; //  4 - 112
 	};
 
 	/* tonemap actually writes to a framebuffer, which is
@@ -141,6 +146,8 @@ public:
 		bool use_debanding = false;
 		Vector2i texture_size;
 		uint32_t view_count = 1;
+
+		bool convert_to_srgb = false;
 	};
 
 	void tonemapper(RID p_source_color, RID p_dst_framebuffer, const TonemapSettings &p_settings);