demos/gears: Add a gears demo.

demos/gears.hlsl

@@ -0,0 +1,55 @@
+cbuffer gear_block : register(b0)
+{
+    float4x4 mvp_matrix;
+    float3x3 normal_matrix;
+};
+
+struct vs_in
+{
+    float4 position : POSITION;
+    float3 normal : NORMAL;
+    float3 diffuse : DIFFUSE;
+    float4 transform : TRANSFORM;
+};
+
+struct vs_out
+{
+    float4 position : SV_POSITION;
+    float4 colour : COLOR;
+};
+
+struct vs_out vs_main(struct vs_in i)
+{
+    const float3 l_pos = float3(5.0, 5.0, 10.0);
+    float3 dir, normal;
+    float4 position;
+    struct vs_out o;
+    float att;
+
+    position.x = i.transform.x * i.position.x - i.transform.y * i.position.y + i.transform.z;
+    position.y = i.transform.x * i.position.y + i.transform.y * i.position.x + i.transform.w;
+    position.zw = i.position.zw;
+
+    o.position = mul(mvp_matrix, position);
+    dir = normalize(l_pos - o.position.xyz / o.position.w);
+
+    normal.x = i.transform.x * i.normal.x - i.transform.y * i.normal.y;
+    normal.y = i.transform.x * i.normal.y + i.transform.y * i.normal.x;
+    normal.z = i.normal.z;
+    att = 0.2 + dot(dir, normalize(mul(normal_matrix, normal)));
+
+    o.colour.xyz = i.diffuse.xyz * att;
+    o.colour.w = 1.0;
+
+    return o;
+}
+
+float4 ps_main_smooth(float4 position : SV_POSITION, float4 colour : COLOR) : SV_TARGET
+{
+    return colour;
+}
+
+float4 ps_main_flat(float4 position : SV_POSITION, nointerpolation float4 colour : COLOR) : SV_TARGET
+{
+    return colour;
+}