By default, internal white texture was used to draw most of the shapes; some time ago, support for white font character from default internal font was added. That way, all basic drawing (shapes, text) could be performed without a texture change and in a single drawing pass.

Now, we move a step further and we allow configuring the texture (and rectangle) used to do the shapes drawing.

By comparing the edges of the rectangles relative to each other we can
determine if they intersect or not.

Note for me: Replace All is NOT your friend...

Library consistency rename... yes, I know, it breaks the API...

Replaced by fabs() that seem to work ok

- Some examples tweaks
- Reviewed ImageFormat()
- Use float for text fontSize

Also added new compilation FLAGS for that pourpose

That way, a user needs only to touch a single file to configure what
features raylib is built with.
Include guards are left out intentionally, because config.h should only
be included in source files, not headers.

Later on, config.h can also define the raylib version (#461).

Using default font texture as base white texture for rectangles reduces
draw calls considerably, actually, raygui can be drawn with a single
pass!

Functionality integrated in DrawRectangle() and selectable with config
flag USE_DEFAULT_FONT_TEXTURE

Variable t was read and modified without interleaving sequence points,
technically undefined behavior. Report by Clang's -Wunsequenced

Added: DrawRectangleGradientV()
Added: DrawRectangleGradientH()

Moved function DrawRectangleT() from text module to shapes
Added some comments

Supports line thickness

Working in modules configuration flags...

Function names, code formatting...

raylib uses spaces between '+' and '-' signs but not between '*' and '/'
signs, it's a chosen convention

To get it, use GetDefaultTexture()

Depth test disabled for 2D and only used on 3D; consequently LINES vs
TRIANGLES vs QUADS buffers drawing order maters... but blending also
works ok.

- Removed DrawQuad() function
- DrawBillboard() uses DrawBillboardRec()
- DrawPlane() uses RL_TRIANGLES
- DrawRectangleV() uses RL_TRIANGLES, that way, [shapes] module uses
only TRIANGLES buffers.