Add low-level hardware instancing support to Meshes.

Add “instancing” Graphics Feature constant.

Add love.graphics.drawInstanced(mesh, instancecount, x, y, …) which draws the mesh multiple times in a single draw call. Each instance of the mesh will appear in the exact same spot unless one of the following are used:

- Add an optional vertex attribute step type argument to Mesh:attachAttribute. “pervertex” is the default. “perinstance” causes the attribute to be per-instance instead of per-vertex, when the mesh is drawn.

- Add love_InstanceID as a built-in read only int variable in GLSL 3 vertex shaders. It is always 0 except when a Mesh is drawn with more than 1 instance specified in the draw call. It can be used to manually compute or index into per-instance values in a shader.

--HG--
branch : minor

src/modules/graphics/opengl/wrap_Graphics.lua

@@ -142,6 +142,7 @@ GLSL.VERTEX = {
 	#define varying out
 	#ifndef LOVE_GLSL1_ON_GLSL3
 		#define love_VertexID gl_VertexID
+		#define love_InstanceID gl_InstanceID
 	#endif
 #endif
 
@@ -221,15 +222,15 @@ vec4 VideoTexel(vec2 texcoords) {
 }]],
 
 	FOOTER = [[
-uniform sampler2D MainTexture;
+uniform sampler2D MainTex;
 void main() {
-	love_PixelColor = effect(VaryingColor, MainTexture, VaryingTexCoord.st, love_PixelCoord);
+	love_PixelColor = effect(VaryingColor, MainTex, VaryingTexCoord.st, love_PixelCoord);
 }]],
 
 	FOOTER_MULTI_CANVAS = [[
-uniform sampler2D MainTexture;
+uniform sampler2D MainTex;
 void main() {
-	effects(VaryingColor, MainTexture, VaryingTexCoord.st, love_PixelCoord);
+	effects(VaryingColor, MainTex, VaryingTexCoord.st, love_PixelCoord);
 }]],
 }