Rewrite terminal.cpp

Rewrites the Terminal class as a GDExtension to be used directly in
Godot without a terminal.gd proxy.

Breaks a lot of things in its current state (e.g. signals and other
functions have not be implemented yet), but does add support for
transparent colors and true color inversion. It also seems to
be about 4x faster (FPS-wise) than the old version with some basic
stress testing.

Old source code has been moved to a different directory to be copied
over and/or rewritten piece by piece.

addons/godot_xterm/shaders/background.gdshader

@@ -0,0 +1,38 @@
+shader_type canvas_item;
+
+uniform int cols;
+uniform int rows;
+uniform vec2 size; // Total size of the Terminal control.
+uniform vec2 cell_size;
+uniform vec4 background;
+uniform sampler2D cell_colors; // Texture containing the cell colors, one pixel per cell.
+
+const int INVERSE_FLAG = 1 << 0;
+// Texture containing the cell attributes encoded using bit flags, one flag per cell in the R channel.
+uniform sampler2D attributes;
+
+void fragment() {
+	// Grid means the part of the terminal that contains the cells.
+	// Will vary depending on font size and the size of the control.
+	vec2 grid_size = vec2(cell_size.x * float(cols), cell_size.y * float(rows));
+
+	if (UV.x * size.x < grid_size.x && UV.y * size.y < grid_size.y) {
+		vec2 grid_uv = UV * size / cell_size;
+
+		int cell_x = int(grid_uv.x);
+		int cell_y = int(grid_uv.y);
+
+		vec2 sample_uv = (vec2(float(cell_x), float(cell_y)) + 0.5) / vec2(float(cols), float(rows));
+		vec4 color = texture(cell_colors, sample_uv);
+
+		int attr_flags = int(texture(attributes, sample_uv).r * 255.0 + 0.5);
+		if ((attr_flags & INVERSE_FLAG) != 0) {
+			color = vec4(1.0 - color.rgb, color.a);
+		}
+
+		COLOR = color;
+	} else {
+		// If outside the grid area, use the background color.
+		COLOR = background;
+	}
+}

addons/godot_xterm/shaders/foreground.gdshader

@@ -0,0 +1,7 @@
+shader_type canvas_item;
+
+void fragment() {
+	COLOR = texture(TEXTURE, UV);
+	// TODO: Check blink attribute and hide/show using the below.
+	//COLOR = vec4(0);
+}