Refactor C++ code

Split features into their own file

Style is becoming less C, more C++

src/common.h

@@ -0,0 +1,46 @@
+#ifndef SHARP_COMMON_H
+#define SHARP_COMMON_H
+
+typedef enum {
+  UNKNOWN,
+  JPEG,
+  PNG,
+  WEBP,
+  TIFF,
+  MAGICK
+} ImageType;
+
+// Filename extension checkers
+bool is_jpeg(std::string const &str);
+bool is_png(std::string const &str);
+bool is_webp(std::string const &str);
+bool is_tiff(std::string const &str);
+
+// How many tasks are in the queue?
+extern volatile int counter_queue;
+
+// How many tasks are being processed?
+extern volatile int counter_process;
+
+/*
+  Initialise a VipsImage from a buffer. Supports JPEG, PNG and WebP.
+  Returns the ImageType detected, if any.
+*/
+ImageType
+sharp_init_image_from_buffer(VipsImage **image, void *buffer, size_t const length, VipsAccess const access);
+
+/*
+  Initialise a VipsImage from a file.
+  Returns the ImageType detected, if any.
+*/
+ImageType
+sharp_init_image_from_file(VipsImage **image, char const *file, VipsAccess const access);
+
+/*
+  Does this image have an alpha channel?
+  Uses colour space interpretation with number of channels to guess this.
+*/
+bool
+sharp_image_has_alpha(VipsImage *image);
+
+#endif