[upstream] Custom text renderer (erincatto/box2d#998)

Added font rendering code independent of imgui.
Converted drawing code to C.
`b2DebugDraw::DrawSegmentFcn` renamed to `b2DebugDraw::DrawLineFcn`.

Author: ikpil

data/font.fs

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+// SPDX-FileCopyrightText: 2025 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330 core
+
+in vec4 color;
+in vec2 uv;
+
+uniform sampler2D FontAtlas;
+
+out vec4 fragColor;
+
+void main()
+{
+	fragColor = vec4(color.rgb, color.a * texture(FontAtlas, uv).r);
+}

data/font.vs

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+// SPDX-FileCopyrightText: 2025 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330 core
+
+layout (location = 0) in vec2 aPosition;
+layout (location = 1) in vec2 aUV;
+layout (location = 2) in vec4 aColor;
+
+out vec4 color;
+out vec2 uv;
+
+uniform mat4 ProjectionMatrix;
+
+void main()
+{
+	gl_Position = ProjectionMatrix * vec4(aPosition, 0.0, 1.0);
+
+	color = aColor;
+	uv = aUV;
+}

data/line.fs

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+// SPDX-FileCopyrightText: 2025 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330
+
+in vec4 f_color;
+out vec4 color;
+
+void main(void)
+{
+    color = f_color;
+}

data/line.vs

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+// SPDX-FileCopyrightText: 2024 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330
+
+uniform mat4 projectionMatrix;
+layout(location = 0) in vec2 v_position;
+layout(location = 1) in vec4 v_color;
+
+out vec4 f_color;
+
+void main(void)
+{
+    f_color = v_color;
+    gl_Position = projectionMatrix * vec4(v_position, 0.0f, 1.0f);
+}

data/point.fs

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+// SPDX-FileCopyrightText: 2025 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330
+
+in vec4 f_color;
+out vec4 color;
+
+void main(void)
+{
+    color = f_color;
+}

data/point.vs

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+// SPDX-FileCopyrightText: 2024 Erin Catto
+// SPDX-License-Identifier: MIT
+
+#version 330
+
+uniform mat4 projectionMatrix;
+layout(location = 0) in vec2 v_position;
+layout(location = 1) in float v_size;
+layout(location = 2) in vec4 v_color;
+
+out vec4 f_color;
+
+void main(void)
+{
+	f_color = v_color;
+	gl_Position = projectionMatrix * vec4(v_position, 0.0f, 1.0f);
+	gl_PointSize = v_size;
+}