mediaapi: Add thumbnail support (#132)

* vendor: Add bimg image processing library

bimg is MIT licensed. It depends on the C library libvips which is LGPL
v2.1+ licensed. libvips must be installed separately.

* mediaapi: Add YAML config file support

* mediaapi: Add thumbnail support

* mediaapi: Add missing thumbnail files

* travis: Add ppa and install libvips-dev

* travis: Another ppa and install libvips-dev attempt

* travis: Add sudo: required for sudo apt* usage

* mediaapi/thumbnailer: Make comparison code more readable

* mediaapi: Simplify logging of thumbnail properties

* mediaapi/thumbnailer: Rename metrics to fitness

Metrics is used in the context of monitoring with Prometheus so renaming
to avoid confusion.

* mediaapi/thumbnailer: Use math.Inf() for max aspect and size

* mediaapi/thumbnailer: Limit number of parallel generators

Fall back to selecting from already-/pre-generated thumbnails or serving
the original.

* mediaapi/thumbnailer: Split bimg code into separate file

* vendor: Add github.com/nfnt/resize pure go image scaler

* mediaapi/thumbnailer: Add nfnt/resize thumbnailer

* travis: Don't install libvips-dev via ppa

* mediaapi: Add notes to README about resizers

* mediaapi: Elaborate on scaling libs in README

media-api-server-config.yaml

@@ -0,0 +1,38 @@
+# The name of the server. This is usually the domain name, e.g 'matrix.org', 'localhost'.
+server_name: "example.com"
+
+# The base path to where the media files will be stored. May be relative or absolute.
+base_path: /var/dendrite/media
+
+# The maximum file size in bytes that is allowed to be stored on this server.
+# Note: if max_file_size_bytes is set to 0, the size is unlimited.
+# Note: if max_file_size_bytes is not set, it will default to 10485760 (10MB)
+max_file_size_bytes: 10485760
+
+# The postgres connection config for connecting to the database e.g a postgres:// URI
+database: "postgres://dendrite:itsasecret@localhost/mediaapi?sslmode=disable"
+
+# Whether to dynamically generate thumbnails on-the-fly if the requested resolution is not already generated
+# NOTE: This is a possible denial-of-service attack vector - use at your own risk
+dynamic_thumbnails: false
+
+# A list of thumbnail sizes to be pre-generated for downloaded remote / uploaded content
+# method is one of crop or scale. If omitted, it will default to scale.
+# crop scales to fill the requested dimensions and crops the excess.
+# scale scales to fit the requested dimensions and one dimension may be smaller than requested.
+thumbnail_sizes:
+- width: 32
+  height: 32
+  method: crop
+- width: 96
+  height: 96
+  method: crop
+- width: 320
+  height: 240
+  method: scale
+- width: 640
+  height: 480
+  method: scale
+- width: 800
+  height: 600
+  method: scale