Add more image generation functions: radial gradient and perlin noise

Add the SaveImageAs function

Add some image generation functions

Namely:
- GenImageHorizontalV
- GenImageHorizontalH
- GenImageChecked
- GenImageWhiteNoise
- GenImageCellular

The gradient implementation may be a bit naive, for example it doesn't do any gamma correction.

Add version info to glfw dependency

glfwSetWindowMonitor and some other functions are only available from 3.2
onwards.

Meson: use gnu99

We use variable init in for loops, so need c99.

Add define to have CLOCK_MONOTONIC work in c99

If we compile with c99 without gnu extensions (gnu99) we need this
define, to have CLOCK_MONOTONIC and similar macros available

Add library versioning

This should always be the number of the raylib release.
Especially when the new version is ABI incompatible the major version
should be bumped.

Initial meson support

Using the library() function instead of shared_library() allows changing
of the library type via the default_library option.
This allows for easy change between static and dynamic library building.

Use 'meson --default-library=static builddir' to build as static, if no builddir yet exists.
Use 'mesonconf -Ddefault_library=static builddir' to change the type for
an existing builddir.

Fix link in readme

Accept external CFLAGS

Some distributions build the programs with a specific set of CFLAGS.
Lets allow that by adding raylibs flags.

Issues when pausin musing and trying to resume (not resuming!)

Setup for a new amazing feature! ;)