Added custom easing example. (#169)

Author: paweljarosz

animation/custom_easing/all.texture_profiles

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+path_settings {
+  path: "**"
+  profile: "Default"
+}
+profiles {
+  name: "Default"
+  platforms {
+    os: OS_ID_GENERIC
+    formats {
+      format: TEXTURE_FORMAT_RGBA
+      compression_level: BEST
+      compression_type: COMPRESSION_TYPE_DEFAULT
+    }
+    mipmaps: false
+    max_texture_size: 0
+    premultiply_alpha: true
+  }
+}

animation/custom_easing/assets/images/logo-color.png

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+images {
+  image: "/assets/images/logo-color.png"
+}
+extrude_borders: 2

animation/custom_easing/assets/sprites.atlas

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+---
+tags: animation, easing
+title: Custom easing - square wave
+brief: This example shows how to define a custom easing curve and use it when animating with `go.animate()` instead of a built-in easing constant.
+author: Defold Foundation
+scripts: custom_easing.script
+---
+
+This example uses the square-wave easing curve. The logo alternates between its starting height and the target height, and the animation system interpolates the positions between the low and high positions when animating.
+
+## Setup
+
+The collection contains one game object with one sprite and one script. The script animates only the `position.y` property with the custom easing.
+
+![setup](setup.png)
+
+## How It Works
+
+The script stores the custom easing samples in a plain Lua table and turns that table into a `vmath.vector`. Defold accepts that vector anywhere `go.animate()` (or `gui.animate()` in case of GUI animations) expects an easing value.
+
+Because the vector is made of repeated blocks of `0` and `1`, the animation keeps snapping between the start value (start y position) and the target value (520 on Y axis, which is above the start value). That makes the easing behave like a square wave instead of a continuous curve. Note that the animation system still interpolates between the frames, so depending on time, the animation can be more trapezoidal or more square wave.

animation/custom_easing/example.md

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+name: "default"
+scale_along_z: 0
+embedded_instances {
+  id: "logo"
+  data: "components {\n"
+  "  id: \"script\"\n"
+  "  component: \"/example/custom_easing.script\"\n"
+  "}\n"
+  "embedded_components {\n"
+  "  id: \"sprite\"\n"
+  "  type: \"sprite\"\n"
+  "  data: \"default_animation: \\\"logo-color\\\"\\n"
+  "material: \\\"/builtins/materials/sprite.material\\\"\\n"
+  "textures {\\n"
+  "  sampler: \\\"texture_sampler\\\"\\n"
+  "  texture: \\\"/assets/sprites.atlas\\\"\\n"
+  "}\\n"
+  "\"\n"
+  "}\n"
+  ""
+  position {
+    x: 360.0
+    y: 200.0
+  }
+}

animation/custom_easing/example/custom_easing.collection

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+local VALUES = { -- <1>
+	0, 0, 0, 0, 0, 0,
+	1, 1, 1, 1, 1, 1,
+	1, 1, 1, 1, 1, 1,
+	0, 0, 0, 0, 0, 0,
+}
+local SQUARE_EASING = vmath.vector(VALUES) -- <2>
+
+function init(self)
+	go.animate(".", "position.y", go.PLAYBACK_LOOP_FORWARD, 520, SQUARE_EASING, 4.0) -- <3>
+end
+
+--[[
+1. This table defines a custom easing curve with normalized samples between 0 and 1. Repeating blocks of zeroes and ones create a square-wave pattern.
+2. Convert the Lua table into a `vmath.vector`, which `go.animate()` accepts as a custom easing value.
+3. Animate the current game object's `position.y` sub-property. The custom curve makes the sprite move between its starting y position and 520 instead with interpolated in-between positions.
+]]

animation/custom_easing/example/custom_easing.script

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+[project]
+title = Custom Easing - Square Wave
+version = 0.1
+developer = Defold Foundation
+
+[bootstrap]
+main_collection = /example/custom_easing.collectionc
+
+[input]
+game_binding = /builtins/input/all.input_bindingc
+repeat_interval = 0.05
+
+[display]
+width = 720
+height = 720
+high_dpi = 1
+
+[physics]
+scale = 0.02
+gravity_y = -500.0
+
+[script]
+shared_state = 1
+
+[collection_proxy]
+max_count = 256
+
+[label]
+subpixels = 1
+
+[sprite]
+subpixels = 1
+max_count = 32765
+
+[windows]
+iap_provider = 
+
+[android]
+package = com.defold.examples
+
+[ios]
+bundle_identifier = com.defold.examples
+
+[osx]
+bundle_identifier = com.defold.examples
+
+[html5]
+show_fullscreen_button = 0
+show_made_with_defold = 0
+scale_mode = no_scale
+heap_size = 64
+
+[graphics]
+texture_profiles = /all.texture_profiles
+
+[collection]
+max_instances = 32765
+
+[particle_fx]
+max_emitter_count = 1024
+
+[render]
+clear_color_blue = 0.183594
+clear_color_green = 0.164063
+clear_color_red = 0.160156
+