gpu/internal/shaders: generate shader variants

We're about to add Direct3D support, where shaders are written in
HLSL. Rather than write shaders twice (or more), convert them to
a GLSL variant understood by the glslcc cross-compiler and generate
the OpenGL ES 2.0 and HLSL variants. The HLSL is used by a future
change.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/stencil.vert

@@ -0,0 +1,53 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision highp float;
+
+layout(binding = 0) uniform Block {
+	vec2 scale;
+	vec2 offset;
+	vec2 pathOffset;
+} uniforms;
+
+layout(location=0) in vec2 corner;
+layout(location=1) in float maxy;
+layout(location=2) in vec2 from;
+layout(location=3) in vec2 ctrl;
+layout(location=4) in vec2 to;
+
+layout(location=0) out vec2 vFrom;
+layout(location=1) out vec2 vCtrl;
+layout(location=2) out vec2 vTo;
+
+void main() {
+	// Add a one pixel overlap so curve quads cover their
+	// entire curves. Could use conservative rasterization
+	// if available.
+	vec2 from = from + uniforms.pathOffset;
+	vec2 ctrl = ctrl + uniforms.pathOffset;
+	vec2 to = to + uniforms.pathOffset;
+	float maxy = maxy + uniforms.pathOffset.y;
+	vec2 pos;
+	if (corner.x > 0.0) {
+		// East.
+		pos.x = max(max(from.x, ctrl.x), to.x)+1.0;
+	} else {
+		// West.
+		pos.x = min(min(from.x, ctrl.x), to.x)-1.0;
+	}
+	if (corner.y > 0.0) {
+		// North.
+		pos.y = maxy + 1.0;
+	} else {
+		// South.
+		pos.y = min(min(from.y, ctrl.y), to.y) - 1.0;
+	}
+	vFrom = from-pos;
+	vCtrl = ctrl-pos;
+	vTo = to-pos;
+    pos *= uniforms.scale;
+    pos += uniforms.offset;
+    gl_Position = vec4(pos, 1, 1);
+}
+